In order to study the influence of chemical heat treatment on the fatigue performance of large bearings, plasma nitriding and rotating bending fatigue test are conducted for the medium alloy Cr and Mo nitriding bearing steel. The middle alloy Cr and Mo bearing steel with double vacuum melting is used as the experimental material to analysis the microstructure after hardening and tempering (930 ℃+550 ℃)and the phase after plasma nitriding. The fatigue morphology and the mechanismroom of crack initiation after the test are analyzed. The relationship between surface defects and fatigue fracture mechanism is established.The results of experimentalshow that the microstructure of the test steel after hardening and tempering is tempered sorbite, M₂C and M₃C is the mainly precipitations. After nitriding,the compound layers in the nitriding layers are mainly composed of γ′phase (Fe₄N) and CrN and the thickness is about 16 μm, which is not conducive to improve the performance of matrix fatigue. The RBF limit of the experimental steel after nitriding is 947 MPa, which is 19.4% higher than the RBF limit of the non-nitriding sample. The cracking of compound layer in the nitrided layer and the cracking of non-metallic inclusions are the two main ways of cracking. The compound layer and surface roughness have great influence on the fatigue life of test steel after nitriding. The results provide experimental and theoretical basis for improving the fatigue performance of medium alloy Cr and Mo nitriding bearing steel.