The digital holographic method is used to characterize the phase modulation depth of phase-only LCOS. Compared with the conventional ways, the digital holographic method could obtain the information around the whole field of view. Besides, the digital holographic method is a non-contact, lossless, high-fidelity way to achieve the phase distribution. In this paper, the lensless Fourier transform digital holography is employed, due to its simple setup and reconstruction process. In LCOS the phase modulation is controlled by displaying the gray level images on its active area. Usually for the phase modulation characterization, the total of all 255 gray level images are displayed in a step change of 10, for each recording. That is why it takes time for the complete calibration. In this method a mask with the entire range of gray-level i.e. from 0-255 is displayed on the LCOS active area and the hologram is recorded, which on reconstruction gives the depth of phase modulation of LCOS for the entire range of gray level. In order to avoid the aberration a double exposure method is used in which two holograms are recorded, one with the 0-255 and other with the zero gray level masks. Also, the sorting by reliability, following a non-continuous path (SNRCP) phase unwrapping algorithm is used for unwrapping the final result. The main advantages of this method are the less number of required recording holograms, the easy and real time calibration. Results are then compared with the conventional method that is young double slit method, which is widely proposed to obtain the phase modulation depth of the LCOS and they are in good agreement with each other. The efficiency of our method is verified by comparison.