Improving actual operation performance of room air conditioners (RAC) shows great importance in the indoor environment and building energy conversation. The installation height and supply air angle of the indoor unit of RAC directly affect thermal comfort and energy performance in heating conditions. In this paper, a combined simulation of building, indoor air distribution, and performance of RAC is proposed to investigate the influence of installation height and supply air angle. The combined simulation model was validated by experiment results. Based on the simulation model, a living room in Shanghai was analyzed. The results indicated that the indoor temperature and the heat transfer coefficient of the external wall vary from different installation heights and supply air angles. When the RAC was installed at a relatively low height, the return air temperature was significantly lower than that in higher height, and the thermal environment was more comfortable. Moreover, the heating capacity of RAC was increased by about 12.2%, and the energy efficiency ratio was increased by approximately 3.5% at the lower height position. The results highlight the issues of installation height and supply air angle, in order to improve indoor thermal comfort and optimize the actual performance of RACs.