A Mask-phase method is proposed to calibrate the modulation property of liquid-crystal-spatial light modulator (LC-SLM) at 561 nm, which is calibrated at 532 nm as ex-factory. Firstly, the relation between the phase contrast of the checkerboard phase grating and the intensity of zero-order diffraction spots is simulated based on Fourier optics. Then the optical path is set up to measure the relation between the gray level and the intensity of zero-order diffraction spots. At last the relation between the gray level and the phase retardation is obtained according to the simulation and experimental results. And the phase-modulation characteristic curve of the LC-SLM at 561 nm is also acquired. After calibration, a spot is modulated by 4λ defocus. In this condition, the intensity distribution deviating from the theoretical value is 45.7, and is reduced by 64.7 compared with 110.4, which is the intensity distribution before calibration. When the spot is modulated by 10λ tilt, the intensities of zero-order and second-order after calibration are reduced to 32.3% and 64.1% respectively, compared to the intensity distribution before calibration. Experimental result indicates that the modulation result of LC-SLM has a great improvement after calibration by the Mask-phase method.