Abstract: The wall-thickness is an important design parameter for two-dimensional ionization chamber dose detector. Under the given geometric parameters of the cavity unit of two-dimensional ionization chamber dose detector, the simulation model was built by using Monte Carlo method. The wall-thickness was calculated with the simulation model using EGSnrc, and the reference numerical value was given. The result shows that when the cavity is a cylinder with diameter of 4.5 mm and height of 5 mm, under radiation of 1.25 MeV, the front wall-thickness of ionization chamber should be more than 4.4 mm and the side wall-thickness should be 1.5 mm in order to meet the demand of accuracy of 1%. Under the radiation with energy of 1.25-4 MeV, the side wall thickness should be more than 5 mm in order to meet the demand of accuracy of 1%. The research result can be used as the design basis for the detector.