Abstract: The output signal of a transverse field gas ionization chamber was simulated by Geant4, Garfield and Magboltz codes. The validity of the simulations was checked by comparing the results of simulation and experiment. In addition, the influence of various gases on the output signal’s amplitude was studied, and the maximum amplitude was obtained with C₄H₁₀ among these gases. This simulation is a good theory tool for improving ionization chamber in the future.