To discharge the chemical laser’s high-speed and low-pressure gas into the background, the start-up pressure determines the recovery pressure of the diffuser. Base on the laser’s actual requirement, the transient modle of the diffuser is established, and the different background pressure’s flow field is obtained with the total pressure 11 kPa on the diffuser’s inlet. The result shows that the diffuser can’t set up successfully when the background pressure increased to 8 kPa. According to the result, a step setup mode is used, the diffuser start up with total pressure 13 kPa, when the flow field is steady, adjusting the inlet’s total pressure to 11 kPa, the diffuser starts up successfully. Experiment is carried out on the DF laser’s test-bed, the result shows that the diffuser’s ability is advanced.