Abstract: Aiming at the problem on alloy compensation in X-ray thickness measurement, based on the energy spectrum equivalent method, according to the one-to-one mapping between alloy compensation coefficient and the energy of single energy ray, and also the mapping between alloy compensation coefficient and thickness of material in the continuous spectrum, an approach transforming the problem of X-ray continuous spectrum into single energy gamma ray was worked out by comparing the alloy compensation coefficients. A concrete proposal including system calibration, thickness calibration, energy calibration and energy spectrum equivalent was proposed. An X-ray thickness measurement simulation model built by Monte Carlo program was established. And the simulation model was used to verify the energy spectrum equivalent result. The results show that the relative errors of thickness measurement compensated by the energy spectrum equivalent for above 90% of alloys are less than 0.1%. It indicates that the alloy compensation approach is practical and it can be used in many fields. The energy spectrum equivalent is reference for studying the hardening of X-ray continuous spectrum.