Abstract: According to several common lifetime calculation methods of radioactive nuclides, the scope of applicable of the four calculation methods, which are named Method of Direct Fitting, Method of Logarithmic Time, Method of Maximum Likelihood and Method of Maximum likelihood when observation time windows is limited, are studied based on the simulation data. As the observation time window is limited or not, the applicable range of the lifetime calculation methods in different observation time windows and different counts are discussed. In simulation, fully stripped ion ^94mRu⁴⁴⁺ was selected as the target nuclide, the lifetime and error in different counts and different observation time windows are obtained, and the applicable range of the four methods is given. The experimental data of ^94mRu⁴⁴⁺ was obtained from the lifetime measurement experiment which is performed by using the Isochronous Mass Spectrometry (IMS) at the HIRFL-CSR facility in Lanzhou. The simulation results are consistent with the experimental results within one error bar, thereby it is further verified the applicable range of the calculation method and the reliability of the simulation data. The simulation results provide theoretical basis and reference for the design of the future lifetime experiments.