Refractive index is the most widely used optical parameter in the optical system, which has an extremely important influence on the optical performance. The optical length consists of refractive index and thickness, which directly affects the time delay characteristics of birefringent devices in the optical system. In this article, a measurement method of refractive index and thickness based on the fiber laser frequency splitting effect is proposed. By rotating the birefringent device inserted into the laser cavity, the birefringent parameters of the device at different angles are measured by using the frequency splitting effect. Based on the refractive index ellipsoid of birefringent device, the relationship among phase delay, refractive index, thickness and rotation angle is established. The refractive index and thickness parameters of the device are achieved by fitting calculation. Experimental results show that the thickness measurement error of birefringent elements is 210 nm, and the intrinsic refractive index measurement error is 10 -5 . It could be widely used in the measurement of intrinsic refractive index and thickness of birefringent elements in the infrared band.