The work aims to research the effects of system leakage for film permeability test on its test results. The mathematical model for different plastic film permeability tests based on differential pressure method was established with the leakage taken or not taken into account. Based on this model, the simulation results by Matlab were given. The simulation results showed that system leakage directly affected the permeability test results of the film, especially for the permeability test of the high barrier films. The small leakage of test system had a great influence on the permeability test results. The objective value of the permeability test is approximately equal to the measured value minus the error caused by system leakage within 24 hours. This provides theoretical basis and reference for the actual film permeability.