In the phosphorescence lifetime measurements with pulse method, both the excitation light pulse parameters and initial time selected for intensity decay analysis are crucial to the accurate lifetime calculation. The relation between phosphorescence intensity decay and the parameters of the excitation light pulse is analyzed based on the linear time invariance. it is found that the phosphorescence intensity decay is independent on the parameters of the excitation light pulse after the excitation light vanishing. If the excitation light pulse vanishing moment is selected as the initial time, the accurate lifetime can be obtained. The experimental results from the Oxy-Phor R2 samples excited by different excitation light pulse have proved the above conclusion.