Abstract: To analyze machining errors of free-form surfaces, a spatial statistical analysis method was presented for the machining errors decomposition, forming systematic errors and random errors. Firstly, the machining errors were analyzed with the method to judge the spatial autocorrelation of these errors. Then a regression model was built based on B-spline surfaces for the deterministic surface. And the adequacy of the regression model was analyzed through calculating the residual errors of all the measured points. Once the residual errors were of spatial independence, the residual errors could be treated as random errors, and the systematic errors were easily obtained. A free-form surface simulation was taken to validate the method. Results show that the error decomposition is accurate and effective. According to the systematic errors of the process system obtained in the surface machining on a NC machining center, the compensation made for the machining error has significantly improved the machining accuracy of free-form surface.