Abstract: 310S stainless steel was multi-micro-alloyed by composite addition of molybdenum, niobium and tungsten, etc., and then treated by thermal aging at different temperatures (650,800℃) for different times (500,1 000,2 000 h) after solution treatment. The effects of thermal aging on microstructure and impact property of the tested steel were studied. The results show that after aging at 650℃, the microstructure of the tested steel consisted of austenite. In addition to M₂₃C₆ phase, there were also Fe₂MoC phase and Cr₇C₃ phase in the second phase. A large number of needle-like second phases precipitated in the grain of the tested steel after aging at 800℃. After aging at different temperatures, the impact absorption energy of the tested steel decreased gradually with the aging time prolonging. Compared with that after 650℃ aging treatment, the impact absorbing energy of the tested steel decreased greatly after aging at 800℃, and the fracture mode changed from ductile fracture into brittle intergranular fracture.