Abstract: Computational fluid dynamics (CFD) was used to analyze the performance of the nuclear main pump gas-liquid two-phase flow under loss of coolant accident. The gas distribution of nuclear main pump, the changing of gas under different gas volume fractions, and the effects of coolant containing gas rate and coolant temperature on nuclear main pump head and efficiency were acquired. The conclusions show that the gas inside nuclear main pump impeller under the accident condition is mainly distributed near the impeller hub area. The gas concentrations gradually increase along the axial direction of impeller, and the gas concentrations gradually reduce along the radial direction of impeller. When coolant containing gas rate is less than 15%, with the increase of coolant containing gas rate, head falls from 113 m to 85 m, efficiency drops from 75% to 65%, but the pump still can work normally. When coolant containing gas rate is more than 15%, the performance of the pump falls sharply, head falls down to 48 m, efficiency falls down to 31%, and the pump fails to work. When the coolant temperature is within 270 ℃ and 350 ℃, efficiency and head seldom change with the increase of coolant temperature, however, when the temperature is more than 350 ℃, the performance of the pump falls sharply and the main pump cannot operate safely.