In development of high-energy chemical laser, the research of diffuser pressure recovery has important engineering application value. In this paper, the diffuser of DF chemical laser is studied by numerical simulation and experiment. The effects of diffuser divergence angle and secondary-throat on diffuser performance are calculated, analyzed and verified by experiments. The results show that the diffuser with 8° divergence angle has weak ability to resist back pressure, and reducing the divergence angle to 5° can effectively improve the ability to resist back pressure. The further optimized supersonic diffuser with secondary-throat can increase the recovery pressure of the diffuser again, reduce the energy loss of the air flow and improve the anti-back pressure characterisitics. At the same time, experimental verification is carried out for different models of diffusers, and the result is consistent with the trend of numerical simulation results.