The formula of harmonic diffraction from a perfect crystal is worked out based on the concept of dynamical diffraction and the spectrum of synchrotron radiation. As an applica-tion, the harmonic diffraction from several commonly used planes of Si crystal is calculated, which reveals that there are more harmonics in the chosen (333) diffraction than the other planes. The data are merely true for the Topography Station at Beijing Synchrotron Radia-tion Lab. (BSRL), Beijing Electron-Positron Collider (BEPC).