By using conventional meteorological observations，ground densified automatic stations，ERA5 （0. 25° ×0. 25°）hourly reanalysis and global topographic data，the differences between the characteristics and causes in snow depth of two extreme heavy snowfall events under the return flow pattern in southeastern Inner Mongolia on November 18-19，2020（Process 1）and November 7-8，2021（Process 2）were compared and analyzed，which can provide the certain references for improving the forecasting ability of heavy snowfall and its impacts in Inner Mongolia. The results show that：（1）The two processes occurred in the same season，with simi‐ lar areas of heavy snowfall and 24-hour cumulative precipitation. Process 1 had a wider influence range，stronger snow intensity and rare freezing rain weather，while Process 2 lasted longer and had deeper snow accumulation， with 11 stations snow depth breaked the historical records.（2）Both Process 1 and Process 2 were caused under the effect of 500 hPa upper-level trough（vortex），700 hPa southwest jet stream，and 925 hPa northeast jet stream. Warm and humid air climbed along the cold air mass at the lower level，producing significant frontogenesis，and the combination of the mid-level front area and the low-level northeast reflux results in the extra large snowfall weather.（3）The intensity of the cold air at the lower level in Process 1 was weaker，the duration was shorter，while the intensity and thickness of the southwest airflow at the mid-level were stronger，resulting in a warmer and more humid mid-level atmosphere and deeper warm layer. The content of liquid water in the clouds at the mid and lower levels was significantly higher than that in Process 2，which conducived to the establishment of a melting layer. Snowflakes or ice crystals melt in the melting layer. In the early stage，it is freezing rain， and in the later stage，it falls to the ground in the form of wet snow. In Process 2，affected by the cold vortex， the system configuration was deeper，the lower-level atmosphere was relatively colder，the cold air mass was stronger and more persistent，which has no melting layer. The temperature conditions are suitable for the formation of flaky snowflakes in the air. After landing，it is mainly dry snow，which is more conducive to the formation of a larger snow depth.（4）At the initial stage of the snowfall，the near-surface temperature in Process 1 was above -1 ℃，and the surface temperature was around 0 ℃. Wet snow could not quickly freeze when it fell to the ground，and it was easy to form freezing rain with the near-surface temperature decreased. In Process 2，the nearsurface temperature was -9 ℃ and the surface temperature was -3 ℃. The near-surface wind speed was weaker， which was conducive to the formation of snow accumulation when relatively dry large snowflakes fell to the ground.