By combining experimental and numerical simulation methods, the structural optimization of the rear deflector for LWQ80 gas turbine flowmeter and the variation law of its metering performance is studied in this article. Based on the investigation of the internal flow field characteristics and flow mechanism of turbine flowmeter, it is found that the separation of boundary layer and the deflection of flow direction are two main causes which influence the pressure loss of the rear deflector. Based on this, the optimization idea of reducing the separation area and improving the diversion effect of the deflector is proposed. An improved structure of rear deflector is designed by extending the length of the rear deflector and posting the position of the deflector. Results show that it is equipped with the selected deflector that the comprehensive performance of the flowmeter is obviously improved. At the detection point of 250 m 3 / h, the pressure loss is reduced by 20. 5% , the constancy of the instrument coefficient is significantly improved, the maximum indication error is reduced by nearly 2. 5 times, and the service life of the flowmeter can be effectively prolonged. Research results are helpful to provide theoretical guidance and technical support for the structure and performance optimization of the gas turbine flowmeter.