In the process of assembly of large aviation composite parts, assembly clearance is inevitable, because the gap is small, when the component is fixed, the internal gap can not reach, so it is difficult to measure. Therefore, in order to overcome this problem, to complete the measurement of small gap in assembly, this paper proposes a method of assembly clearance measurement using reverse reconstruction and virtual assembly. Based on the background of the large aircraft composite parts measurement, combined with the principle of binocular vision measurement, this paper combines the measurement of vision and laser, studying the technology from point cloud obtained to the reconstruction of assembly simulation model, measuring the assembly gap by CATIA.