Numerical simulations of a canned motor pump at partial load condition have been conducted for studying the strong rotor–stator interaction. SST k−ω turbulence model with wall function was adopted for solving three-dimensional (3D) unsteady Reynolds-averaged Naiver–Stokes (URANS) equations. The inner flow field for four typical rotor positions, where the head changes significantly, was explored. It has been shown that when blade approached tongue region, the radial velocity decreased where the pressure at suction side was bigger than pressure side. As a result, a negative work was exerted on the fluid which led to minimum work done by the impeller. At this condition, maximum work was resulted in due to the upstream blade. The fluid from balance hole affected the main flow near hub region which caused a vortex near channel pressure side. Due to impeller channel rotation through tongue, the vortex was enlarged which contributed separation flow at impeller inlet region. The separated flow caused pressure reduction in the channel. Static pressure at impeller outlet near shroud side was always greater than at near hub side which resulted in negative radial velocity at impeller outlet near shroud side. Due to the blade accession to the tongue, the static pressure at impeller channel increased meanwhile the area characterized by negative radial velocity enlarged.