The strut structure directly affects the flow-field characteristics and aerodynamic performance of the gas turbine exhaust diffuser. Based on verifying the reliability of the numerical method, the exhaust diffuser with different struts was numerically calculated by solving three-dimensional Reynolds-averaged Navier–Stokes. The flow process and flow loss mechanism in the diffuser were analyzed, the influence of two different structures of tapered struts on the aerodynamic performance of the exhaust diffuser under different inlet preswirls was explored, and the aerodynamic performance of the exhaust diffuser with tapered struts was compared with a conventional exhaust diffuser with linear struts. The results show that, compared with the conventional linear strut, under different inlet preswirls, two different tapered struts can both weaken the flow separation in the exhaust diffuser, thereby reducing the total pressure loss. When the inlet preswirl is greater than 0.35, the total pressure loss coefficient of the exhaust diffuser with structure-C tapered struts decreases by up to 0.07. The two types of tapered struts also change the flow structure at the exhaust diffuser outlet, which affects the uniformity of the outlet airflow, and then affect the static pressure recovery coefficient. Under different inlet preswirls, two types of tapered struts can be effective to increase the static pressure recovery coefficient of the exhaust diffuser, for the exhaust diffuser with structure-C tapered struts, the static pressure recovery coefficient can be increased by up to 0.065, relative increase of 20%. The research in this paper shows that the tapered structure can significantly improve the aerodynamic performance of the exhaust diffuser under different inlet preswirls.