Form gear grinding technology has characters of high-accuracy,it’s grinding wheel can be modified flexibly,and replaces generating method gradually. A piecewise modification method of cycloidal gear was designed based on form gear grinding technology. This method used angel modification in working section and spline curves in other spaces. The isometric and shift combination modification method can not fit angel modification curve completely and dose not meet the conjugate mesh condition.Our method overcomes these disadvantages. Both methods were applied in the example of modification cycloidal gear in RV-20 E reduce. The results show the combination modification method has a high error relative to the objective curve. The piecewise modification method meets the conjugate mesh condition completely.