In this article, the structure of the TM80 gas turbine flowmeter is optimized by using numerical simulation and experimental test. The numerical simulation results reveal that the sudden change of pressure gradient and boundary layer separation are mainly caused by the core support and rear deflector. Therefore, the structural optimization schemes about the slope of core support and the diameter of rear deflector are proposed. Specifically, the slope of the movement support is designed to be 15°. The step flow on the side of the movement support is transformed into a gradual flow, the diameter of the rear guide fluid is reduced to 62 mm, and the step flow on the side of the rear guide fluid is transformed into a pipe flow of equal diameter. The numerical and experimental results show that when the slope of core support is 15° and the diameter of rear deflector is 62 mm, the pressure loss of the flowmeter is significantly reduced, the instrument coefficient becomes more stable, and the linearity error becomes obviously smaller, which verify that the structural optimization schemes can greatly improve the metering performance of the flowmeter. The research results are helpful to provide theoretical guidance and technical support for the development of gas turbine flowmeter with better performance in the future.