Electrical submersible pumps (ESPs) widely used in oil-gas artificial lift consume a lot of electric energy in long-term operation. This paper mainly focuses on the gas–liquid performance and predicting shaft power and efficiency of a 25-stage ESP. First, the calculation methods of two-phase hydraulic parameters and corresponding dimensionless hydraulic coefficients based on isothermal compression are proposed. Ignoring the gas compressibility will result in large errors in calculating two-phase hydraulic parameters. Then, the effects of liquid flowrate, inlet gas volume fraction, and rotational speed on head, shaft power, and efficiency are analyzed. The severe two-phase head degradation disappears in downstream stages of the ESP because of the decreasing interstage gas volume fraction. Similar to the head, the shaft power and efficiency decrease slowly at first, then rapidly, and finally slowly with the increase of inlet gas volume fraction. Finally, correlations are proposed for predicting the shaft power and efficiency by the dimensionless head and flow coefficients. There is a power function relation between two-phase head coefficient and efficiency. Thus, through the pump head which can be easily acquired by differential pressure signals in pipeline, prediction correlations for shaft power and efficiency are established with the relative errors lower than 10%. The prediction method based on two-phase dimensionless coefficients can also be referenced to ESPs with different types.