To design a SX-700 monochromator for Shanghai Synchrotron Radiation Facility (SSRF) and to enable its main design index, the rotation angle repeatability of scanning photon energy,to be less than 0.43″, the error sources affecting the rotation angle repeatability of the grating sine mechanism are investigated and then the grating sine mechanism is simulated numerically by using the finite element software ANSYS. The errors for the rotation-angle-repeatability of the grating sine mechanism are analyzed according to the results of the numerical simulation, which shows that the rotation-angle-repeatability is 0.28″.Based on the analysis,a SX-700 monochromator is designed,and the angle repeatability of the grating sine mechanism is measured with an autocollimator system designed by us.Measured results indicate that the rotation angle repeatability of the designed monochromator is 0.15″,which meets the requirements of the grating sine mechanism for the rotation-angle-repeatability.