Multistage space guide vane long shaft pump is widely used in many important fields because of its compact structure, convenient maintenance and simple pipeline. However, due to the complex flow phenomenon in its channel, the space guide vane long shaft pump has the problem of low performance. This paper takes xb4.3/240-300lc vertical long shaft fire pump as the research object. When the blade outlet width is the same, three schemes of impeller outlet inclination angle (0°, 15°and 25°) are designed, and CFX is used for numerical simulation to analyse the difference of internal flow field of each scheme. The results show that compared with the other two schemes, the scheme with an inclination angle of 15° has better performance in efficiency( η ) and head( H ). The pressure on the working surface of the blade is higher than that on the back of the blade, and the pressure near the front cover plate increases with the growth of the inclination angle. Under the condition of small flow, the velocity at the impeller outlet increases with the growth of inclination angle. Under the condition of large flow, the velocity at the impeller outlet decreases with the growth of inclination angle. Similarly, the turbulent kinetic energy shares the same pattern with the velocity at the impeller outlet. Under all working conditions, small vortex structure appears in impeller and guide vane. It is found that the scheme with impeller outlet inclination angle of 15° has smaller impeller vortex structure under standard working condition.