Abstract: Efficiency calibration is one of the key problems to be solved in the activity detection of radioactive waste barrels.Based on the assumption of uniform distribution of media material and radionuclides contained in the barrel, Monte Carlo method was used to simulate line attenuation coefficients and body source detection efficiency under various energies and various sample densities.The body source detection efficiency function with linear attenuation coefficient and energy as independent variables is obtained by multivariate nonlinear fitting.And, based on the idea of shell source method, the experiment of the homogenous body source is approximately replaced by multiple point source combination measurement.Three kinds of energy ray and three kinds of density samples have been verified experimentally, and the detection efficiency is calculated respectively by the method of Monte Carlo simulation combining with numerical fitting and the method of experimental measurement.Then, the respective activity estimation results are obtained by solving the efficiency matrix equation, and, the relative error is less than 30%.The simulation results are basically consistent with the experimental result, which proves the effectiveness of the method.