The basic principles of wavefront sensing technology based on phase diversity (PD) are discussed. The noise adaptability of the PD technique and the influence of the detector defocus distances error on the accuracy are analyzed. PD technology is applied to assisted align off-axis and coaxial three-mirror reflecting optical system, and the wavefront maps of multiple fields of view are compared with interferometric test results. Experiment results show that wavefront sensing and assisted alignment of three-mirror reflecting optical system can be realized by using PD technology. Compared with the results of interferometric test, results of wavefront sensing are all better than 0.02λ RMS. The accuracy can satisfy the requirement of engineering.