Abstract: The interference of coincidence summing of γ-ray cascade radiation in gamma spectrometry can cause the increase or decrease of full energy peak count, therefore the deviation of the measurement result is from the true value. From the principle of coincidence summing, the numerical expression of the correction in γ-ray cascade radiation was introduced by using the case of ¹³⁴Cs radionuclide as an example. By the stimulation of full energy peak efficiency and total efficiency based on the Monte Carlo calculation program MCNP, the accuracy of the numerical expression was verified by experiment and calculation. Finally, the factor of coincidence summing correction for the key radionuclides released from the nuclear power plants was presented through the calculation method. The calculation results indicate that the coincidence summing interference for ¹¹⁰Ag^m and ¹³⁴Cs in aerosol and biological sample is more than 20%, for soil and water with relatively low detection efficiency can reach 10%, and other nuclides also have varying degrees of interference caused by coincidence summing. The result can provide technical reference for the accurate measurement and the uncertainty evaluation in γ-ray spectrometry.