The Liquid Metal cooled Reactors (LMRs) are one of the potential technologies being considered by the Generation IV International Forum (GIF). Accurate measurement of coolant flow in reactor fuel bundles is crucial for optimizing the safety and performance of nuclear reactors. However, traditional industrial sensors have certain limitations when directly measuring liquid metal flow, especially under ultra-low flow conditions that may occur during accident scenarios. In this study, a flow meter based on the energy balance principle was initially designed. The mass flow rate is obtained by heating the coolant and measuring the temperatures at the inlet and outlet of the flow meter. The flow meter is designed based on basic conservation laws, allowing for precise measurement without the need for complex calibration. Due to the temperature stratification phenomenon of liquid metal at high temperatures and low flow rates, a mixing strategy based on the Venturi effect is proposed to obtain a representative temperature after heating. Numerical simulations are performed using evaluation criteria related to the mixing effect. Through the analysis of the flow field and temperature field, the impact of this mixing strategy on flow characteristics and temperature uniformity is explored. The results show that the proposed mixing strategy can achieve temperature uniformity in liquid metal under ultra-low flow conditions within a short distance. The flow meter proposed in this study provides an effective solution for accurately measuring the flow rate in LMRs cooling systems under high-temperature and ultra-low flow conditions, overcoming the measurement accuracy limitations of traditional flow meters at ultra-low flow rates.