Non-intrusive load monitoring（NILM） is one of the most promising technologies for load monitoring. Different classes of loads have unique observable transient profiles during the turn on/off process. With these transient profiles， NILM will be able to overcome the limitations of the algorithm in identifying the operation of the loads with the steady state load information， realizing decomposition and analysis of the cluster loads in the entire substation or a building. NILM should be able to capture the transients automatically and trigger the related program to save the transient data for further analysis. A novel sliding window version two-sided cumulative sum（CUSUM） algorithm is proposed for detecting the transients. Simulation results show the feasibility of the algorithm proposed.