The initial phase and wavelength response of the spatial light modulator (SLM) will be shifted with the working time and the environment variation. To solve this problem, we study the initial phase calibration and the multi-wavelength response of phase-only SLM. Four-step phase shifting interferometry is used to measure and calibrate the initial phase of SLM. Michelson interferometer is used to study the relation between the order of interference fringes and the gray scale of different wavelengths experimentally. It is shown that the initial phase is calibrated appropriately and the correction coefficient of wavelength response is achieved successfully. This investigation will provide technologic reference for using SLM in various environments.