In order to improve the application level of GM counter, the detection efficiency simulation program of GJ4105 GM counter was designed based on Geant4 software. The correctness of the simulation program is verified by the measured sensitivity, and the deviation between the calculated sensitivity based on the simulated detection efficiency and the measured sensitivity is only 0.5%. The results show that the simulation results of Geant4 are reliable, which can provide an effective and convenient simulation method for the design of GM counter.