Abstract: 40CrMnNiMo structural steel was subjected to 900 ℃×60 min quenching+600 ℃×120 min tempering+different time cryogenic tests. The effects of different cryogenic time on the microstructure and mechanical properties of 40CrMnNiMo steel were studied by means of SEM, torsion test, impact test and hardness test. The results show that comp ared with the quenching and tempering treated 40CrMnNiMo steel, the quenching and tempering treated 40CrMnNiMo steel with different cryogenic time has finer carbides and more uniform and compact structures, and the microstructure of 40CrMnNiMo steel after cryogenic treatment for 15 h has the smallest and most uniform carbides. With the extension of cryogenic time, the torsional strength, hardness and impact absorbed energy of 40CrMnNiMo steel all first increase and then decrease, and the torsional strength and hardness of 40CrMnNiMo steel reach the maximum value of 982 MPa and 38.1 HRC respectively, after cryogenic treatment for 15 h, the maximum impact absorption energy of 40CrMnNiMo steel after cryogenic treatment for 5 h reaches 72 J.