Abstract: In order to improve the hydraulic performance of the mixed-flow pump in off-design condition and master the basic principle of inlet guide vane regulating condition, authors conducted the numerical simulation for internal flow field of mixed-flow pump in different operating conditions by using finite element analysis software. With the modification of setting angle of the inlet guide vane, the change regulation of absolute flow angle, ralative flow angle and circumferential component of absolute velocity in impeller inlet were analyzed. The results showed that absolute flow angle was less than setting angle, and the smaller the flow was, the larger the difference was, and the effect of inlet guide vane regulation under large flow condition was more obvious than under small flow condition. The results also indicated that the inlet liquid flow can satisfy non-impact or smaller incidence angle inlet conditions in certain flow extent, and the pump can be operated with high efficiency in off-design conditon.