Previous research studies focused only on data of local air temperature and humidity, ignoring the water body itself, which cannot definitively answer the question of how the Three Gorges Reservoir's (TGR) water affects the local climate overall. To understand the effect of the TGR on the local climate quantitatively, this paper provides an original mathematical hypothesis and proves in theory there is only one way to calculate the transfer of heat and humidity between the TGR and the local air. Based on this mathematical hypothesis, a detailed research method to explore the effects of the TGR's heat and humidity on local climate was formed. A field investigation was conducted and a research site was selected in Chongqing. This study has determined the effects of the TGR's heating or cooling on the air during the measuring period. A mathematical model to assess the effects of heat and humidity from the TGR on local climate was set up. The final results based on the mathematical model show that the average air temperature decreased 0.67 K and the average moisture content increased 0.25 g/kg during the 24 hours measuring time for the area studied.