The crystal rotation method is usually used to measure the pretilt angle of liquid crystal molecules, but it is unsuitable to measure the pretilt angle of 20°～70°. In order to expand the range of applicability of crystal rotation method, we propose the pretilt angle measurement method to use prisms to assist the crystal rotation. A liquid crystal cell is located between two prisms, and the incident light enters from the former prism, passes through the crystal cell and exits from the back prism. We will get the incident angle on condition that the birefringence retardation is zero and the light transmission is maximal, and obtain the pretilt angle with optical geometry. The numerical simulation and experimental results show a feasibility of this method.