Research results ： According to the characteristics of urban mass transit construction and fluctuant property of passenger traffic volume, this paper analyzes the content of the post - assessment of financial benefit and presents a stochastic gray systemic model for the post - assessment of financial benefit. Application of the GM (1,1) model forecasts and fits the passenger traffic volume to obtain passenger flow data as the basic data for the post -assessment of financial benefit so as to research the theory and method for post - assessment of financial benefit of urban mass transit construction. The calculation result shows that the GM (1,1) model can better indicate the change rules of passenger traffic volume with good extrapolating property and precision and can be applied in the post-assessment of financial benefit of urban rail transit construction.