Influenced by the cannabis legalization of USA and EU, the illegal indoor cannabis cultivation got popular in China in recent years. Since indoor cultivation of cannabis has a large number of electricity consumption with distinct feature of electricity usage, A dectecting methed based on the relative entropy of the frequency distribution of electricity usage is developed to identify abnormal users in this paper. Firstly, the electricity usage data of indoor cultivate cannabis is generated from the laboratary simulation. Then, the electricity usage feature difference between the indoor cultivation of cannabis users and the normal residential users are compared in terms of the temporal, frequency domain and specific indicitors. Thereafter, the identification process of obnormal users of indoor cultivation of cannabis is proposed. In the end, a numerical analysis of electricity usage data from Irish residents suggests that the proposed approach can seperate residential users and anomaly users. It is helpful to identify the illegal user with the indoor cultivating cannabis.