Abstract: A rare extreme precipitation event (maximum cumulative rainfall of 562.5 mm in 24 h) was experienced in western Guangdong on 22 June 2017, which resulted in a number of local historical rainfall records and caused serious property damage. This paper analyzed the maintenance mechanism of this extreme precipitation in the warm zone using the observation data of the automatic regional station and the Doppler radar and wind profiler. The microphysical characteristics of precipitation were studied using a two-dimensional video disdrometer (2DVD) and an FY-3D microwave humidity sensor. Results show that a surface mesoscale convergence line that triggered the convective cells on the night of 21 June leads to the generation and maintenance of the quasi-stationary long-life MCS (Mesoscale Convective System). The convergence line near the Tianlu Mountain lies between the cooler air reactor (lower equivalent potential temperature) caused by the evaporation and cooling of precipitation and the warmer air reactor (higher equivalent potential temperature) from the ocean. The temperature difference between the two sides of the mesoscale convergence line is about 2°C–3°C. The ambient atmosphere is characterized by the weak convection inhibition energy and low uplift condensation height. The height of the convective cloud on top of this precipitation event is relatively high, and the center of the extreme value of precipitation is located in the large value region of the bright temperature gradient that lies to the west of the transition zone between the two bright temperature extreme value centers. The precipitation raindrop spectrum shows the characteristics of warm precipitation in the high-humidity environment. During the period of the strongest precipitation, the concentration of small raindrops exceeds 10⁵ mm⁻¹ m⁻³, which is much higher than the average value in south China during summer (about 10⁴ mm⁻¹ m^-3). Few large particles also exist, leading to a higher precipitation efficiency and local heavy precipitation. The cold outflow of the northerly wind generated by the MCS encounters the warm and wet airflow of southwest China and forms a strong convergence, which continuously triggers new convective cells, enabling the MCS to remain quasi-static and continuously produce precipitation.