Abstract: The plasma environment which creates Xray laser brings the difficulty to the application of Xray multilayer reflector .The most difficult is that the Xray emitted by the plasma destroys and damages the multilayer reflector. It is shown by the simulated test that the damage d threshold of Mo/Si multilayer reflector made in china is less than 0.10J/cm 2. With the exposure the radiation dose the roodmean square roughness of multilayer apparently increases and the reflectivity almost drops to zero. Accor ding to these results,the article state some ideas for preventing damages of the multilayer reflector.