Abstract: The thermal power accuracy of HPR1000 was analyzed, and the contribution of steam generator outlet pressure measurement accuracy, feed water temperature measurement accuracy and feed water flow measurement accuracy to HPR1000 thermal power accuracy was calculated. The quantitative data proves that the main feedwater flow measurement accuracy has the greatest influence on the thermal power calculation accuracy. Considering the current problem of low accuracy and the deterioration of the orifice flowmeter due to long-term use, a high-precision (0.3%) ultrasonic flowmeter is proposed to measure the main feed water flow. The calculation shows that the ultrasonic flowmeter can achieve a power increase of 0.97%.