σ phase is one of the main precipitates affecting the toughness of austenitic stainless steel, V-notch impact test, SEM, EDS and TEM analysis were conducted on the newly developed 22Cr15Ni3.5Cu stainless steel after 650°C aging. Precipitation mechanism of σ phase and its effect on the toughness of the material were analyzed. The test results show that toughness of the material decreases to 25.6J after 300h aging, σ phase started to precipitate along the grain boundary after 500h aging, and in the crystal after 1000h aging. The precipitation spacing is about 100 nm, forming a gradually increasing size from crystal to grain boundary. As the precipitation time 500h of σ phase was later than the critical aging time of ductile brittle transition, it can be inferred from the test result that σ phase is not the main precipitation phase affecting the toughness of 22Cr15Ni3.5Cu.