This paper studies the four-quadrant homologous characteristic and analyzes the gas–liquid two-phase flow head degradation of ACP100 reactor coolant pump. The theoretical basis and the operating conditions of the pump's four-quadrant homologous characteristic are introduced. To verify the reliability of the computational fluid dynamics (CFD), the comparison of the results from CFD and test is implemented, and with these datum for the pump is plotted with homologous curves. In this paper, homologous pump characteristics are used to make the proper estimation on complete characteristic outside the testing range. Meanwhile, a combined method of impeller Euler head distribution and entropy generation rate analysis are applied for the head degradation of ACP100 reactor coolant pump under gas–liquid two-phase flow condition, and 70% of gas void fraction (GVF) is shown as the maximum degradation point. In addition, head degradation multiplier plots homologous curves are used to predict the homologous characteristic of specific GVF.