A new numerical simulation method according to distorted crystal optical theory was used to predict the direct-cooling crystal monochromator optical properties(crystal Darwin width) in this study.The finite element analysis software was used to calculate the deformated displacements of DCM crystal and to get the local reciprocal lattice vector of distorted crystal.The broadening of direct-cooling crystal Darwin width in meridional direction was estimated at 4.12 μrad.The result agrees well with the experimental data of 5 μrad

while it was 3.89 μrad by traditional calcultion method of root mean square(RMS) of the slope error in the center line of footprint.The new method provides important theoretical support for designing and processing of monochromator crystal for synchrotron radiation beamline.