The multiple exposure method has been proposed to fabricate combined diffractive optical element,but the multiple etch processing is complex and large alignment error will be introduced. According to error-diffusion method in computer-generated hologram (CGH) and partial coherent imaging theory, a new method to design coding mask of profile-combined element with error-diffusion method is presented. The designed coding mask for color-separation grating and beam-sampling grating (CSG-BSG) combined optical element with error-diffusion method is illustrated , and the intensity distribution of the spatial image in partial coherent imaging system is simulated, compared with ideal combined element relief. It shows that mean-square error is 7.5% and volume error is 10.2% after correction.