To improve the maturity of the containment leakage rate measurement technology, this study evaluates the applicability and reliability of the theoretical model of constant pressure method for containment leakage rate. Two theoretical analysis models, namely the standard condition mean method and the operating condition mean method were proposed. The validation of the theoretical models requires the acquisition of gas parameters within the containment under a constant internal pressure environment, followed by the calculation of the leakage rate. Firstly, sensors were arranged in the simulation body of steel containment with a free volume of 1 000 m³ to monitor the gas status, and a flow regulation device was designed in the inflation pipe to maintain constant pressure. Experiments were carried out in the environment of positive pressure and negative pressure. The stability and consistency of the containment leakage rate calculated by means of standard conditions and working conditions were explored. Secondly, the data from engineering test of pressure drop method in a nuclear power plant were analyzed by using the theoretical model of constant pressure method. The applicability of the theoretical model of constant pressure method to engineering scale containment and its compatibility with pressure drop methods were explored. The study indicates that the two theoretical analysis models have good consistency in calculating the containment leakage rate, with a relative deviation of less than ± 0.2%. The leakage rate calculated by the theoretical model of constant pressure method can achieve the accuracy of the traditional pressure drop method, with a relative deviation of less than 5% between the two methods. Therefore, the theoretical model of constant pressure method can be used to analyze the test data of pressure drop method. The findings of this research can provide support for the theoretical research and engineering application of the constant pressure method for containment leakage rate technology.