Abstract: An early warning method for the risk of crowd gathering based on the bi-directional flow ratio of the section was proposed according to the characteristics of the opposite movement of pedestrian flow at the business street. Its error was analyzed in theory and verified by an example.It is found that using this method as a criterion of crowded events is feasible.Whether the measured values are greater or less than the true values,the error of the ratio between the inward and the outward flow across the section is less than that from a pedestrian flow statistic system,so the method is helpful for improving the accuracy of crowd statistic tools to some extent.In addition,the method can be used as a reference in selecting the type of a pedestrian flow statistic system according to requirements for the measuring accuracy of actual applications.