To study the pressure recovery ability of the diffuser of chemical lasers, an equipment that can simulate different background pressure is designed. The simulated background pressure can be changed by changing the valve’s pass area. The influence of the straight part’s length before the valve being considered, different length models are simulated and the validation test is carried out, whose result is consistent with that of the simulation. The result shows the feasibility of simulating different background pressure with a flashboard valve. The equipment will redound to the investigation of the diffuser’s recovery pressure in chemical lasers.