The problems of self-excitation and worsening of S-Parameter caused by structure of microwave active module always happen in the design of microwave circuits. Up to now, there is no complete theory to support the design of cavity. This study presented a theoretical analysis of the cavity effect of microwave module, in which microwave was transferred by microstrip lines. The microwave module with microstrip lines was simulated by High Frequency Structure Simulator(HFSS), a powerful full-wave simulation tool. A microwave module of X-band was designed according to the results of simulation and measurements of S-Parameters. The measured results agree well with the simulated results.