The current implementation of the synchro to digital conversion (SDC) can not directly detect harmonic which can cause the problem of low precision angle measurement. To improve accuracy of angle, harmonic interference was analyzed and summarized in the axis-angle to digital conversion process. Through detailed research, a new angle measurement formula is proposed in this paper, this formula can be very good for angle error correction and compensation. In order to meet the practical requirements, a software measurement method based on FFT algorithm of harmonic analysis and error correction is put forward. Matlab simulation shows that the formula and method is effective and feasible.