Abstract: Fast SAGD start−up enhancement technique injects the high−pressure steam in short time to build a high−temperature steam chamber with high porosity and permeability. In this study, downhole microseismic monitoring was performed in a fast SAGD start−up enhancement well group of heavy oil reservoir in Fengcheng oil field to delineate the geometric and dynamic propagation characteristics of formed steam chamber. Time reversed imaging method was applied to obtain the weak event locations with high accuracy. The quantitative estimation of length, width, height, azimuth and volume of the steam chamber was also estimated based on the event locations. The variation of steam chamber geometric and energy distribution with dates were also analyzed. The propagation characteristics of steam chamber front in depth direction respect to heterogeneous interlayer was also studied. The microseismic monitoring results demonstrates that the injection well and production well has been connected and the expansion area has partially extended to the bottom of the oil layer interface. In addition, amount of microseismic events were found above the injection well which suggests the steam has break the heterogeneous interlayer. Therefore, the reservoir has been successfully stimulated. Finally, the fitting relationship between steam chamber geometry characteristics and injection dates has been obtained based on the microseismic monitoring results, which provides significantly theoretical and practical foundation for the design and optimization of fast SAGD start−up enhancement in the studied area.