The cash flow method is widely used in gas well investment evaluation, which has the disadvantages of long evaluation cycle and lag of evaluation results, and is not conducive to timely deployment and adjustment.In order to overcome this problem, this paper focuses on the wells internal revenue rate and calculation method in Sichuan basin, and defines a new parameter: the unit investment of absolute open flow. The parameter is proved that it has normal distribution relationship with the wells efficiency.So then, the probability model is presented about gas wells efficiency through the unit investment of absolute open flow. Because of the model, gas well efficiency evaluation is completed immediately after the well testing.The model and method presented in this paper have a guiding significance for the gas well investment and the development of gas field.