Abstract: Multistage centrifugal mixed-flow pumps are commonly used to transport high-pressure gas-liquid two-phase fluids. However, due to gas-phase compressibility and the slip velocity between phases, gas pockets tend to form in the impellers, leading to uneven flow distribution and inconsistent pressure-boosting capabilities across stages. This study experimentally investigates the influence of inlet gas volume fraction (IGVF) on the inter-stage characteristics of 3-stage and 25-stage centrifugal pumps. Additionally, numerical simulations based on the Euler-Euler inhomogeneous flow model, accounting for gas compressibility, were conducted for the 3-stage pump. The results show that under pure water conditions, the pressure rise across each impeller is nearly identical. Under gas-liquid conditions, the overall pressure-boosting capacity decreases significantly with increasing IGVF, with secondary and final-stage impellers outperforming the first stage. Gas accumulation intensifies in the impeller passages, spreading along the suction side toward the outlet. Velocity threshold analysis reveals that at low thresholds, the velocity-area ratio decreases stage by stage, while at medium thresholds, it increases progressively, indicating enhanced impeller pressurization. Furthermore, pulsating entropy generation losses are primarily concentrated near the impeller and diffuser inlet regions. This study provides a theoretical basis for optimizing the design of multistage mixed-flow pumps.