Abstract: In the process of gamma ray dose rate screening, the calculation from the activity limit of radionuclides to the dose rate limit is an important part. The balance of decay series of ²³²Th, ²³⁸U and ²³⁵U in rare earth tailings is broken during smelting, which makes the calculation of on-site gamma-ray dose rate of rare earth tailings more complicated. The on-site gamma-ray dose rate calculation method is based on the decay law and chemical characteristics of nuclides, and the gamma-ray dosimetry of ²³²Th, ²³⁸U and ²³⁵U natural radioactive series is processed in stages, and the gamma-ray dosimetry of the whole decay series is obtained by calculating the gamma-ray dosimetry of each decay sub-series. In the closed environment, the main radiation subsystems of ²³²Th, ²³⁸U and ²³⁵U series have the characteristics of reaching the decay equilibrium in a short time, so as to avoid the problem that the activity of the nuclides of each generation of the decay series varies with time in the non-equilibrium state and reduce the calculation amount. Based on the monte carlo method, the relationship between the specific activity of each decay subsystem and the field gamma dose rate was established. Then, the gamma-ray dose rate can be calculated if the specific activity of each radionuclide is measured in the laboratory.