Abstract: Using the NCEP/NCAR （National Centers for Environmental Prediction/National Center for Atmospheric Research）daily reanalysis and Guangdong daily precipitation datasets from 1961 to 2017 and selecting and classifying the rainstorm processes in the warm sector in Guangdong only caused by South China Sea monsoon trough without the frontal system, the temporal and spatial distribution characteristics of different types of monsoon trough rainstorm processes and their relationship with atmospheric low-frequency oscillations are investigated, focusing on revealing the atmospheric circulation field evolution characteristics in different phases of quasi-biweekly oscillations and the source and mechanism of the low-frequency signals of the monsoon trough rainstorm process of the (strong) southwest monsoon type and using the synthetic analysis, wavelet analysis, and Lanczos filtering methods. The results showed that (1) the monsoon trough rainstorm process of the (strong) southwest monsoon type accounted for 54.1% of the total cases, mainly occurring in the three major rainstorm centers (i.e., Yangjiang–Yangchun–Enping, Haifeng–Lufeng–Shanwei, and Longmen–Fogang) along the coast and inland of Guangdong. The other three types of monsoon trough rainstorm processes (i.e., (strong) southwest monsoon convergence type, southeast monsoon disturbance type, and tropical cyclone away/dissipation type) mainly occurred in the two major rainstorm centers along the coast of Guangdong and on both sides of the Pearl River Estuary. (2) The monsoon trough rainstorm process mainly exhibits quasi-weekly (5–8 days) and quasi-biweekly (10–24 days) oscillations but less than 32–65 days oscillations. (3) During the intermittent start–peak–weak period of the monsoon trough rainstorm process of the (strong) southwest monsoon type, the low-frequency signal not only originates from the southeastward propagation of the quasi-barotropic low-frequency wave train consisting of a series of anomalous anticyclones and cyclones from Western Europe to Northeast Asia but also originates from the northwestward propagation of low-frequency cyclones in the tropical western Pacific of southeast Philippines in the middle and lower layers of the troposphere, inducing the South Asian High to move westward from Bangladesh to Pakistan and gradually strengthening and extending the western Pacific subtropical high northwestward from Balkashi Lake to Baikal lake. Meanwhile, central and eastern China is controlled by low-frequency anticyclonic circulation or high-pressure ridge in intermittent periods gradually turning into the low-frequency cyclonic circulation or upper trough in the peak period. (4) In South China, the strength of the upper-air divergence gradually increases, as the strong northeasterly at the southeast side of the low-frequency anticyclonic circulation changes to the strong westerly from the bottom of low-frequency cyclonic circulation that controls North China and South of the Yangtze and the northwesterly from the periphery of the low-frequency anticyclone that controls the Indian Peninsula–Northwest South China Sea. The middle and lower layers are controlled by the low-frequency anticyclone circulation or high-pressure ridge gradually turning into the (strong) southwesterly, which comes from the north of the low-frequency anticyclone that controls the central and northern South China Sea and the Bay of Bengal. Meanwhile, the strong low-frequency cyclonic circulation and positive vorticity are formed over Southwest China, South of the Yangtze, and South China when the strong southwesterly is blocked by the low-frequency anticyclone circulation over Northeast Asia, Japan, and the southwestern Pacific. Guangdong is located in the confluence area of the ascending motion from the monsoon circulation circle at the low latitude, and the secondary vertical circulation circle at the middle latitude, and the strength of the southerly wind anomaly from the middle and lower layers of the troposphere and the ascending motion and absolute vorticity gradually increase. The sea-level pressure gradually changed from a weak uniform pressure field to a closed low pressure extending northward from the Beibu Gulf. The South China Sea monsoon trough is lifted northward and strengthens, making the monsoon trough rainstorm process of the (strong) southwest monsoon type shift from the intermittent period to the peak period. These phenomena are beneficial to medium-range and extended-range forecasts of monsoon trough rainstorm processes.