Luminescence decay is an important process of luminescence. It is important to measure the lifetime for the study of luminous mechanism. But there are two flaws in the traditional research on the concepts and methods: firstly in terms of the concepts, luminescence decay is considered as the reduction in the number of the excited states and ignored as the decline of the luminescence intensity, so they are different concepts. Secondly the methods are based on the change rule of the excited states and the boundary conditions without the support of the experiments do not conform to the reality. The traditional method is demanding as to the equipment and photoluminescence is necessary. In order to correct these two flaws and reduce the cost, a new electroluminescence decay measurement system is set up , which could be used for all periodic luminescence. Based on the principle of energy conversion, we use periodic luminescence method, and pulse interval time as a time scale to measure the lifetime easily. The luminescence intensity shows different rules through the comparison between the pulse interval time and the lifetime. A new luminescence measurement system is proposed based on this theory. The experimental result shows that the luminescence intensity keeps invariant and then gradually decline with the increase of excited frequency. We successfully measure the lifetime by measuring the inflexion point and also found a positive correlation between initial luminescence intensity and lifetime. We believe that lifetime is the change of the luminescence intensity. This is a great innovation different from the tradition research on the number of the excited states. At the same time, through the experiment it is proved that the lifetime is related to the initial intensity and increase the understanding of the lifetime.