To elucidate the relationship between the gas-liquid two-phase flow pattern and performance in a centrifugal pump, this study investigates the energy and transient characteristics of the pump under various inlet gas volume fraction (IGVF) and liquid flow rate (LFR) through visualization testing. The results indicate that at the nominal flow rate, the critical gas content for transitioning between different flow patterns increases with the LFR. At lower flow rate, the transition from gas pocket flow (GPF) to gas-liquid separation flow (GLSF) shows the most significant gradient change in critical gas content. At higher flow rate, the transition from bubbly flow (BF) to GPF exhibits the most significant gradient change in critical gas content. When the flow regime is BF at the nominal flow rate, additional gas entry leads to a decrease in pump head and efficiency, accompanied by periodic changes in pressure fluctuation. When the flow pattern shifts to GPF, additional gas entry similarly decreases pump head and efficiency. Notably, pump head decreases significantly at the IGVF of 3.1% and 3.9%, with reductions of 52.8% and 69.0%, respectively. Unlike BF, pressure fluctuation does not exhibit apparent periodicity. Under the condition of IGVF = 1.0%, as the flow regime transitions from BF to GPF, increasing the LFR results in a decrease in pump head and an increase in efficiency, along with an increase in the peak value of pressure fluctuation. When the flow regime in the pump is BF, both pump head and efficiency decrease with increasing LFR, leading to a decrease in the peak value of pressure fluctuation. Notably, when the LFR increases to 35 m³/h, the continuous increase in IGVF results in a maximum reduction in pump head and efficiency of 35.56% and 20.04%, respectively.