Abstract: Abstract: The unsteady gas-liquid two-phase flow has often posed a great threat to the pump performance, particularly with the increase of inlet gas void fraction during transportation. The traditional two-fluid model cannot consider the variation of bubble diameter and the interaction between bubbles. In this study, a group equilibrium model (Musig model) was proposed to simulate the internal flow in the centrifugal pump with the semi-open impeller under different inlet gas void fractions. The simulation works were then verified by the experimental pump performance and visualization test. Some parameters were obtained, including the pump performance, the bubble distribution in the middle section, and the turbulent kinetic energy distribution under different inlet void fractions at the design flow rate. A proper cause was analyzed for the deterioration of pump performance under a large void fraction, together with the velocity distribution and bubble variation at the tip clearance between blade and pump casing. The results show that the maximum handing ability was 4.6% about the inlet gas void fraction of the model pump at 1000 r/min. When the void fraction was greater than 3%, the Musig model was more accurate to predicate the pump performance, particularly suitable for the evolution of gas-liquid two-phase flow, such as bubble morphology, fragmentation, and polymerization, compared with the Euler-Euler two fluid model. Specifically, the maximum errors of head coefficient and efficiency were 1.6% and 5%, respectively, when the inlet gas void fraction was 4% at a pump design flow rate. Moreover, the predicted bubble distribution and flow pattern were consistent with the visualization experiment. The flow patterns (such as the uniform/polymeric bubble, cavitation, and separated flow) gradually appeared in the impeller and volute channels, with the increase of inlet gas void fraction. There was a mainly uniform distribution of bubble flow inside the channel at a design flow rate when the inlet gas void fraction was less than 1%. The polymeric bubble flow appeared, when the inlet gas void fraction reached 3%. The cavitation flow then dominated, when the inlet gas void fraction reached 4%. Further, the separate flow appeared, when the inlet gas void fraction reached 4.2%, where the flow channel was gradually blocked. The tip clearance was an important parameter to determine the distribution of gas-liquid two-phase flow pattern in the pump, which promoted the bubbles' transposition from the blade pressure side to the suction side. The maximum velocity was distributed at the gap between the impeller outlet and the volute, where was the place with the most bubble aggregation and distribution patterns of gas-liquid two-phase flow. There were the increasing large-scale vortex and outlet reflux in the impeller channel, with the increase of inlet gas void fraction, leading to the large distribution of turbulent kinetic energy in the high void fraction area, where the unstable flow was intensified inside the pump, and finally leading to the pump idling after the inlet gas void fraction reached 4.6%. This finding can also provide a sound reference for the comprehensive analysis of unsteady flow characteristics in a centrifugal pump.