How to evaluate crowd safety in crowded areas is a tough, but important, problem. According to accident-causing theory, uncontrolled release of hazardous energy among overcrowded pedestrians is the basic cause of crowd disaster. Therefore, crowd energy is modeled in this paper, which takes both pedestrian kinetic energy, pedestrian potential energy and pedestrian internal energy into consideration. Furthermore, the crowd energy is discussed in an empirical study of subway station based on videos. The result shows that the crowd energy can be used to evaluate crowd safety performance.