Abstract: In the second loop of the nuclear power plants, the coolant mass flow is one of the critical parameters for calculating thermal power and also determining the control of the steam generator. Generally, Venturi flowmeter is used to measure the main feedwater flow in nuclear power plants for its high accuracy, wide application, simple construction, and reliability. However, when measuring the low mass flow, the Venturi flowmeter shows an obvious fluctuation, which would result in a significant measurement error, and serious influence on the normal operation control of nuclear power plants. In order to satisfy the need of high accuracy in measuring the coolant mass flow under a wider mass flow range, it is essential to figure out the possible reasons for the measurement error and propose an accurate manufacturing technology to improve the measurement accuracy of Venturi flowmeter. In this paper, through modling the full-size Venturi flowmeter and its upstream and downstream pipelines, the measurement results of the Venturi flowmeter and orifice flowmeter were compared based on the theoretical analysis and numerical analysis. It is found that the measurement results of Venturi flowmeter fluctuate significantly in the low mass flow stage. Based on the measurement principle of differential pressure flowmeter, it is deduced through theoretical analysis that the main reason for the error of Venturi flowmeter in turbulent pulsating flow is that the derivative term in the flow equation is omitted when the differential pressure data is converted into flow output. Based on the analysis of the flow field in the Venturi flowmeter, the vortex distribution in the upstream venturi is significantly improved, and the stability of the Venturi flowmeter is effectively reduced. Through computational fluid dynamics (CFD) numerical simulation, a detailed comparative analysis was carried out to investigate the characteristics of flow stability and resistance before and after optimization. The comparison results of the measured mass flow show that the flow adjustment equipment can improve the stability in mass flow measurement. The feasibility of the optimized design scheme is demonstrated. In addition, the numerical study was carried out to investigate the pressure loss characteristics of Venturi flowmeter integrating different typical types of flow adjustment equipment, including Gallagher, K-Lab, NEL (Spearman), and Zanker. The numerical results show that the resistance of K-Lab flow adjustment equipment is minor. The scheme proposed in this paper can effectively improve the measurement accuracy of Venturi flowmeter and guide the performance improvement of Venturi flowmeter in engineering applications.