Abstract: Rational determination of production capacity is one of the technical problems during turning open pit into underground mining. With Shirengou Iron Mine as the object, seven projects feasible in technology were brought forward in combination with its specific situation. Considering the integral effect of technical, economical and social factors on the production capacity and according to mining codes, standards and regulations, it is obtained by fuzzy comprehensive evaluation and analytic hierarchy process that the best production capacity of Shirengou Iron Mine during turning open pit into underground mining is 2 150 000 t·a^-1.