Abstract: To solve the problem of alloy compensation for high accuracy radiographic inspection technique, a calculation formula of alloy compensation coefficient was presented and the relation between alloy compensation coefficient and thickness was found. The alloy compensation coefficient measured by an isotope thickness measurement instrument indicates that the theoretical analysis results are correct. A geometric model was built by MCNP5 based on the Monte Carlo method. The simulation results are consistent with the theoretical analysis results. It proves that the model is accurate and the method to calculate the alloy compensation coefficient is feasible. To compensate the experimental and simulated thickness with the calculated alloy compensation coefficient, the relative measurement error of 304 stainless steel decreases from about 1% to 0.1% at 1.25 MeV.