The surface modificative layer microstructure, residual stress and microhardness of carbonitriding M50NiL steel after different grinding parameters were studied. The results show that grinding surface modificative layer is very small deformation martensite and some austenite which is nanometer grain zone. The grain turning phenomenon is apparent. The content of austenite is affected by grinding heat of different grinding parameters. The grinding surface modificative layer may be divided into three parts, the outermost grinding heat affected layer about 0–20 μm, the middle high hardness surface modificative layer about 20–70 μm and the internal diffusion surface modificative layer about 70–200 μm. In order to reduce the impact of different grinding parameters and improve the structure property of surface modificative layer, some grinding surface modificative layer may be removed but keep the maximum depth of not more than grinding heat affected layer and then the high mechanical properties of M50NiL steel are obtained.