Numerical simulations were conducted on a three-stage helicon-axial multiphase pump under different conditions of gas volume fraction (GVF). Water was chosen as the first phase and ideal air was the second phase. k-ω SST turbulence model and dispersed phase zero equation model were selected to simulate the liquid-gas two-phase steady and unsteady flow. The hydraulic performance of the pump according to the simulations was well coincident with the experimental results. The results show that GVF and total volume flow declined smoothly from the first stage to the third for the compressiblity of gas. Due to different centrifugal forces on the two phases, the gas was distributed mainly around the hub; with the increase of GVF, the differential pressure of the first stage decreased slowly while the second and third stagesincreased at first and then decreased. When the inlet GVF was less than 10% or more than 80% , the differential pressure among the three stages were close, so the design and simulation in these conditions could regard as the imcompressible fluid. But in other conditions, it was necessary to consider the fluid as the compressible one; because of the stator-rotor interference and limited number of blades, the average pressure decreased at the interface between impeller and diffuser.