A method used for harmonic forecasting and warning of active distribution network is put forward. Considering harmonic change caused by intermittent energy based on the principle of random sum, it forecasts the harmonic of active distribution network after intermittent energy inputting or cutting out from the network. So it will be used as the basis of coordinated control of the new energy for active distribution network. Considering the harmonic changes caused by intermittent energy inputting active distribution network is big, it adopts the cloud model to model the harmonic current, and uses the entropy of cloud model to measure the harmonic current fluctuation range of normal operation mode, so as to determine the intermittent energy grid and harmonic current anomaly threshold. The harmonic current abnormal or not is judged by the comparison between measured data and harmonic anomaly threshold, and so the harmonic warning is realized. Simulation and engineering examples verify the practicability and effectiveness of the proposed method.