Abstract: Exit width of impeller has a vital impact on the performance of forestry fire pump.The optimum exit width of impeller was determined through numerical calculation in this paper. The exit width of impeller was optimized by means of PRO/E while the inlet and outlet blade angle, inlet and outlet diameter and other parameters kept unchanged. The exit width of impeller was set 8, 10 and 12 mm and then the performance of the forestry fire pump was simulated with respect to different exit widths of impeller by means of CFD(computational fluid dynamics),in an effort to obtain the optimal exit width for large head and large flow pump. It was calculated that exit width of impeller of 10 mm was the optimum dimension, with which the velocity and pressure fields of inner fluid were well distributed and can reach as high as 40 m/s and 2.0×10⁵ Pa, and the optimum working efficiency could reach 91.72%. Experimental test in field confirmed pump with exit width of impeller of 10 mm could meet the requirement in terms of the head and the flow.