Due to environmental illumination and other factors, actual shearing interference image used to measure the rotation angle contains high-frequency speckle noises. According to the characteristics of shearing interference images, a low-pass filter based on homomorphic filtering principle is selected to filter these images. The desired filtering result can be achieved by adjusting the filter parameters such as the cutoff frequency radius and the order of the filter. The Hilditch thinning algorithm is applied to extract the center of the fringes to calculate the spacing between neighbor fringes. The accuracy of the algorithm is verified by the actual and simulation experiments with the error of fringe spacing detection less than 0.01%.