The method to calculate the spatial coherhence of synchrotron radiation soft X ray has been put forward. The spatial coherence has been calculated numericaly according to the data from a NSRL soft X ray zone plate pinhole monochrometor at three working wavelengths. In addition, a formula to calculate the diameter of the coherent area has been concluded. This should be helpful to the soft X ray experiments and the refinement of the monochrometor at NSRL.