To improve the heat exchange of air-cooled condenser unit (ACCU), the numerical simulations of ACCU with a new swirling device (named swirler) installed at outlet of the fan (improved air-cooled condenser unit, named IACCU) is executed, under various vane angles of different swirlers compared to the same conventional air-cooled condenser unit (CACCU) without swirler. The swirl intensity theory is presented, and the flow fields and temperature distributions of the IACCUs are obtained and compared with that of the CACCU. The simulated results show that the heat transfer characteristic and the flow field characteristic of the IACCU are both better than that of the CACCU. The vane angle of the swirler is optimized for the IACCUs with four different kinds of swirlers, and the most effective one of these devices is found out.