Abstract: Tissue equivalent proportional counter (TEPC) is widely used for microdosimetric measurements in heavy ion beams， which provides basic data for calculating relative biological effectiveness (RBE). To study the influence of TEPC wall on microdosimetric measurements and RBE calculations of heavy ion beams, an Ideal-TEPC combined with Monte Carlo (MC) simulation was introduced to accurately calculate the microdosimetric quantities and RBE at different water equivalent depths of carbon ion beams. The results using Ideal-TEPC and general TEPC show that the TEPC wall produces serious irradiation field distortions for the carbon ion beams, and leads to deviation for the results of TEPC measurements and the RBE calculations. The deviation increases with the penetration depth. Treating the TEPC wall as a water layer with a fixed water equivalent thickness could numerically offset the influence of the TEPC wall on the RBE calculation, but over-correction occurs at the distal end of the spread-out Bragg peak (SOBP). Using Ideal-TEPC combined with MC simulation, the influence of wall effect and irradiation field distortions caused by TEPC wall on microdosimetric measurements and RBE calculations of heavy ion beam can be effectively avoided.