In traditional phase retrieval method based on transport of intensity equation (TIE), there are problems of slow speed and low accuracy caused by moving the object to be measured or CCD in the process of light intensity images acquisition. In order to solve these problems, a new method using the liquid crystal on silicon (LCOS) as tunable-lens for phase retrieval is proposed in this paper. Firstly, the phase distribution images with different focal lengths are loaded in a LCOS to make it realize the function of tunable-lens. Then the light intensity images of different defocusing distances are collected in the same imaging plane by the designed system in this paper. Finally, the phase information is obtained by solving the TIE. With this method, different defocusing images can be formed by changing the phase distribution image loaded in the LCOS, avoiding the errors caused by mechanical shift. The correctness and effectiveness of the proposed phase retrieval method are verified by the simulation and experimental results.