Abstract: The helium mass spectrum sniffing probe method is widely used in the process of the single point leak testing for the spacecraft. In this paper a mathematical model of the sniffing probe method is built and based on the model, the quantitative relationship between the leak rate of the testing object and the displayed value of the leak detector is obtained. This model is then validated by a great number of tests. Finally, the theoretical basis of the sniffing probe comparison method is analyzed. The results show that the sniffing probe method is reliable for measuring the single point leak rate.