Abstract: Based on the principle of ellipsometry and the 4×4 matrix method, a way of measuring the parameters of anisotropic films whose optical axis is parallel to the surface with the single-wavelength ellipsometer is proposed, the parameters including birefringence, thickness and Euler angle. Two sets of ellipsometric parameters can be obtained by rotating the sample at 90°, and four film parameters of the anisotropic film can be obtained with the inversion algorithm to invert two sets of ellipsometric parameters.The effects of incident angle, film thickness, Euler angle and its rotation error on measurement accuracy are discussed with numerical simulation. The ellipsometric parameters of the polyimide film whose optical axis parallels its surface are measured and inverted. The results show that the algorithm has good stability and high precision and the ordinary refractive index, extraordinary refractive index, thickness and Euler angle are 0.000 1, 0.000 1, 0.1 nm and 0.03° respectively. The maximum measurement errors of ordinary refractive index, extraordinary refractive index and thickness are 0.001 2, 0.004 4 and 4.57 nm respectively. This method has good measurement stability, self-consistency and reliability as well as the advantages of simple measurement process and low requirement for instruments. This method extends the measurement range of single-wavelength ellipsometer and the way of measuring anisotropic film, so it has practical significance.