Abstract: The spatiotemporal distribution of Mesoscale Convective Systems (MCSs) in Northeast China and their formation mechanisms are investigated using GPM satellite observations and ERA5 reanalysis data from June to August over 2001—2020. The results indicate that MCSs predominantly initiate near mountainous such as the Greater and Lesser Khingan mountains and the Changbai mountains, with the genesis frequency significantly and positively correlated with terrain slope. The MCSs precipitation contribution is most prominent over plain areas. On average, 7.71 MCSs occur per summer over the Northeast plain, with a precipitation intensity (2.45 mm/h) significantly higher than the climatological mean. MCSs contribute an average of 33.56% to extreme precipitation (≥10 mm/h), with a maximum contribution of approximately 70%. The number of MCSs affecting the Northeast plain exhibits a significant increasing trend (0.43 events/a), primarily driven by the increase in MCSs generated near the Greater Khingan mountains. MCSs generated near the Greater Khingan mountains exhibit longer durations (16.4 h) and higher precipitation intensities (5.1 mm/h) compared to those from other areas. The formation of MCSs is jointly influenced by anomalous circulation and orographic lifting. Upper-level divergence ahead of cyclonic circulation anomalies, low-level southwesterly moisture transportation, and an unstable stratification with cold upper and warm lower layers provide favorable conditions for the development of MCSs.