As the development of cities becomes more quick,  the accidents happened in public venues resulted form massing crowd become more and more.  This paper uses social risk to quantify the risk. On one hand,  in terms of former research outcome,  the paper deems that the occurrences of the accidents in public venues,  in a time span, can be described as Poisson distribution, and then, the quantitative model of accident occurrence probabilities can be reasonal out. On the other hand, through the sum of the occurrences of accidents of different severity level, which is to embody the situational probability of accidents of different severity level with its frequencies,  the outcome of accidents mortalities will be obtained,  which is used to figure out the probabilities of the accidents of different casualty numbers. In the end,  the F - N curve will be achieved. To use the F - N curve,  it is able to analyse the social risk of crowd massing venues.  Taking some statistical accidents as references,  it describes how to use the model.  The result proves the model is reasonable and accurate to a certain extent.