Taking TRZ80 gas turbine flowmeter as the research object, the structural optimization scheme of the front rectifier and the rear deflector is proposed by combining numerical simulation and experimental test. Through the analysis of the internal flow field characteristics before and after structural optimization, the exact hydrodynamic mechanism behind the structure and performance optimization of flowmeter is revealed. Research results show that the sudden change of pressure drop in the front rectifier and rear deflector area, as well as the vortex structure and reflux phenomenon at the rear deflector tail, are the main mechanisms that affect the metering performance of the flowmeter. The optimized structure can greatly reduce the sudden change of pressure drop, vortex structure and reflux phenomenon. In addition, the optimized structure can significantly reduce the pressure loss of the flowmeter. And it can also obviously improve the measurement accuracy and stability of the flowmeter. The pressure loss and linearity error are reduced by about 48. 58% and 32. 43% , respectively. Research results could help to provide theoretical guidance and technical support for the development and mass production of gas turbine flowmeter with better metering performance in the future.