Two air conditioning systems are equipped in a hotel lobby. It is found from the field test that the actual air rate is 40% and 16% of the nominal value, respectively, of the two systems, which is far lower than the design requirement. The air rate of the outlets varies greatly, and the coefficient of uniformity is 129.1% and 111.6% respectively of the two systems. Air distribution in the lobby is bad and thermal comfort is poor. Moreover, sharp reduction of return air makes portions of fresh air increase, which will lead to high energy consumption. Reconstruction is carried out to improve the thermal environment with the assistance of the CFD method. First, the original system is simulated by CFD method to verify the CFD method and propose modification suggestions. Then air conditioning load and air rate of the lobby is recalculated and duct redesigned. Simulation results show that the air distribution and thermal comfort of the improved scheme can meet the design requirement. The reconstructed system has been running for about two years and has shown good performance.