The muon is the main part of cosmic rays which can be detected on the Earth’s surface and an elementary particle of standard model. The muon decays to an electron and neutrinos. The lifetime measurement of muon can help students to know the decay properties of muon and also the time dilation effect of special relativity. In this paper，a NaI( Tl) detector is used to detect the signals of stopped muon and its decay electron. A simplified experimental method is introduced to measure the mean lifetime of muon in the laboratory