Abstract: The helico-axial multiphase pump is often used for gas-liquid mixture transportation in offshore oilfields, and slug flow is the main reason for the unstable operation of the pump. Aimed for slug flow condition, a self-designed three-stage multiphase pump is set to the object to perform unsteady simulations and fluid-structure interaction calculations, and the inlet gas void fraction (IGVF) is set from 20% to 80%. The results show that affected by the flow from the slug, the gas-liquid two-phase flow pattern in the multiphase pump changes sharply, resulting in severe fluctuations in the differential pressure, spindle torque and deformation of the multiphase pump. The gas-phase enters the first-impeller along the suction blade surface when affected by Taylor bubbles, while the second and third-stage impellers gas-phases are in the form of small air masses flow into the impeller along the pressure blade surface. The deformation trend of impeller torque, differential pressure and the main pump spindle is similar to that of trigonometric function, while the fluctuation of torque is more intense, and the shape variable of spindle increases with the inflow of liquid plug, and the maximum deformation amount increases by 10.9% at high GVF relative to IGVF.