A method for improving machining precision of CNC spiral bevel gear grinding machine was investigated. The tooth surface of spiral bevel gears was discretized and a method for calculating the position vector and normal vector of discrete points was developed according to the mathematical model of the tooth surface. The relation between proportional change parameters and tooth form errors caused by machining errors was analyzed. Based on the proportional change parameters obtained from machine settings calculation and freewill modification characteristics of grinding wheel, a method for compensating machining errors was proposed. By applying optimization algorithm, the coefficients of proportional change parameters and the variation of grinding wheel parameters were computed. The proposed method was validated via practical machining on YK20100 grinding machine and inspecting on M&M Sigma 7 gear measuring instrument. The results show that this method can reduce the tooth form errors within 5.4μm. 