Abstract: This study aims to explore the gas-liquid two-phase flows during the self-priming process in an external-mixing self-priming centrifugal pump using Euler-Euler modeling, with an emphasis on the transient flow characteristics and the underlying mechanisms for the oscillating of the gas-liquid interface in the suction pipe. A homogeneous multiphase model (without considering the interphase velocity slip), and the standard k-ε turbulence model were adopted in the simulation. The opening boundary conditions were applied for the fluid at the inlet and outlet of the calculation domain, in order to capture the rising process of liquid level in the suction pipe. The speed was measured to determine the rotating speed of the rotor. The speed acceleration was then performed on the self-priming during the start-up process of the pump. The transient flow fields of the main components were investigated to reveal the gross features of the gas-liquid interface rising in the suction pipe, the mass flow rate of the liquid at the impeller inlet, the average gas volume fractions of pump components, and gas-liquid distribution in the flow passage of the pump. The simulated rising process of the gas-liquid interface in the suction pipe was agreed well with the experimental measurements. It infers that the opening boundary conditions were suitable for the reverse flow in the suction pipe caused by pressure oscillation in the impeller. The transient liquid phase flow rate at the impeller inlet indicated that the self-priming process was divided into three stages, namely the rapid absorbing, the oscillating exhaust, and the accelerating exhaust stage. The duration of the three stages accounted for 11.7%, 61.3% and 27% of the total self-priming time, respectively. The impeller rotating speed increased sharply at the rapid absorbing stage, where the initially stored liquid in the suction chamber was quickly sucked into the impeller to discharge the gas in the impeller. The oscillating exhaust was the main stage of the self-priming process, where the liquid in the gas-liquid separation chamber repeatedly entered the volute and the vicinity of the impeller outlet through the reflux hole in the process of gas-liquid mixing and gas exhausting. There was a periodic change with 0.25 s in the volume fraction of gas in the main pump components and the oscillation of liquid level in the suction pipe. But the inertia of the liquid column caused a phasegap between the liquid level oscillation and the pressure fluctuation at the impeller entrance. The oscillation amplitude of the gas-liquid interface increased with the increase of liquid column height. The accelerating exhaust stage was the final stage of the self-priming process, where the liquid in the suction pipe rapidly entered the impeller and the gas in the pump was quickly discharged. The finding can also provide an important reference for the design and performance optimization of self-priming centrifugal pumps.