Objective Quartz crystal is an important birefringence material, which is widely used in optical related fields. The two main optical parameters of quartz crystal wave plate are phase retardation and fast axis azimuth. Due to the influence of manufacturing process, these two actual parameters will deviate from the theoretical value, so it is usually necessary to accurately measure the optical parameters before use. Ellipsometry is usually used to measure the parameters of quartz crystal in a wide spectrum , but the existing ellipsometric measuring instruments often assume that the optical axis of the crystal is aligned with the measuring optical path, which introduces measurement error, especially in the ultraviolet band. Therefore, it is necessary to propose a fitting model for accurate