In the running process of cloud computing system, the idle compute nodes will generate a large amount of idle energy consumption. Furthermore, the unmatching task scheduling strategy will also cause a great waste of energy consumption. This paper presents a policy of energy optimal management for cloud computing system based on task scheduling strategy. First, use queueing system to model the cloud computing system for analyzing the mean response time, mean power consumption of cloud computing system, and constructing the energy consumption model of cloud computing system. In order to reduce waste of energy, a high service utilization task scheduling and a low execution energy task scheduling strategy are propsed, which are used to reduce idle energy and “luxury” energy respectively. Based on the idea of the strategies, an algorithm is designed which is called minimum expectation execution energy with performance constraints (ME³PC). Repeated experiments show that this energy management strategy can reduce the energy consumption considerably while meeting performance constraints.