Torsion test is an important means of determination engineering materials mechanical properties, a high accuracy twist angle measurement method is designed. The twist angle signal is acquired through grating and infrared photocell by Moiré fringe, then interpolation twist angle signal with high-frequency pulses, so as to convert it to pulses, the twist angle is received by processing pulses. This measurement system combined with FPGA and SCM, the function of FPGA are generating pulses、interpolating twist angle signal and counting the pulses, the result of count is processed by SCM. The twist angle accuracy is 0.002°.According to the experiment results, and the main errors of this system are received, they are signal processing error、electric conversion error and concentricity error. The errors sources are analysed, disposal methods for the errors sources are presented.