Abstract: The mechanisms involved in the development of high temperature anomalies in Northeast China during the summer of 2018 were studied using observational and reanalysis data. First, daily temperatures recorded at observation stations in the region throughout the summer were analyzed. Next, the excess heat factor index of the observation stations was calculated. July and August were the main anomaly high temperature periods when high temperature anomalies occurred in the southern part of Northeast China. The South Asia high (SAH) and western Pacific subtropical high (WPSH) were significantly intensified during this period, and overlapped with each other on different levels and extended northward. There was also an increase in the negative vorticity anomalies in the overlapping area of the SAH and WPSH, and the two northward extending systems continued to drive the negative vorticity anomalies. In addition, an abnormal down draft occurred over the southern part of Northeast China together with sinking adiabatic warming and clear-sky radiation warming, which may have been important factors involved in surface warming in this area. Furthermore, surface temperature anomalies were significantly correlated with negative vorticity anomalies at geopotential heights from 300 to 500 hPa during the summer of 2018 in this region. It was also determined that the quasi-stationary Rossby wave energy propagation in the summer subtropical westerly jet was closely related to the anomalous enhancement of the SAH and WPSH. Significant simultaneous warming of the western Pacific warm pool during the summer also promoted unusually strong convective activity in the Philippines. The Pacific–Japan (PJ) wave train was excited at a geopotential height field of 500 hPa, which also led to the enhancement and northward extension of the WPSH. In summary, the existence of the SAH and WPSH and their overlapping were the main causes of the high temperature anomalies in the southern part of Northeast China during July and August 2018.