As conventional phase shifting interferometers can not test the wavefront aberration from a large diameter and long optical path system accuracely, a new commonpath shearing interferometer is designed to elimnate the system error introduced by the standard wavefront. The new design takes an agglutination prism consisting of a triangular prism and a rhombic prism as a shear device to produce interference by the wavefront under-test with its replication. A group of different relative aperture lens switching devices are used to control the shearing ratio of the shearing interferometer. Compared with the data from 4D dynamic interferometers under different temperature conditions,it shows that the repeatability of designed shearing interferometer is about λ/80 and the accuracy of root mean square is better than λ/80(λ=630 nm). The design of the commonpath shearing interferometer greatly reduces its dimension and weight and also improves the experiment efficiency for long optical path interference measurement.