Abstract: In order to ensure the quasi-determined value of the filter box source activity of the ¹³³Ba and to improve the accuracy of the iodine monitor detection efficiency scale, high-quality coconut shell activated carbon with a particle size of 20 to 40 mesh and a bulk density of 0.47 g/cm³ was used. The self-absorption correction of the thickness of different activated carbon layers in the body was studied. Using the MCNP Monte Carlo simulation program, the detection efficiency ε_t of the surface source at different thicknesses t of the activated carbon layer and the detection efficiency ε₀ of the unfilled medium were calculated. The self-absorption correction curve F（t） was established with the thickness of the activated carbon layer. And experiments verify the validity of the simulation results. Through simulated calculations and experiments, the self-absorption correction factor F（t） of the activated carbon under different activated carbon layer thickness was obtained. The results show that the difference between the simulation calculation and the experimental results is about 1%. For the 356 keV γ energy point of ¹³³Ba, the self-absorption correction factor F（t） of the activated carbon in the filter cartridge is obtained, and the technical method of the activated carbon filter cartridge originating from the absorption correction is given. F（t） is used to calculate the self-absorption correction of self-made ¹³³Ba activated carbon filter body. Comparing with the experimental results, the two are within 2%. Therefore, the self-absorption correction of the activated carbon cartridge body can be performed using the experimentally obtained self-absorption correction curve F（t）.