In order to assess crowd risk involved with mass of people gathering in large gathering spots, four common crowd flows including two-way flow, heterogeneous crowd flow, circular flow and arching phenomenon were specifically analyzed on the basis of investigation of crowd's route selection. The formation mechanism of crowd risk consists of four stages namely free movement, retention, congestion and stampede, and by using the method of event tree, the important impact of crowd flow on crowd risk was found out. Combined with crowd risk model, the thinking of computer simulation on crowd risk was proposed to provide a reference for crowd security management in large gathering spots.