The effect of thermal discomfort on human well-being and performance was studied in the field office, and an attempt was made to elucidate its psychological mechanism. Thirty participants were recruited to perform subjective evaluations and performance tests under 5 different conditions (25 °C, 27 °C, 29 °C, 31 °C, 33 °C). During the experiment, the air temperature was considered as an independent variable and other parameters were kept at the same level. The results show that thermal discomfort can lead to poor comfort and reduced performance, and people report that many sick building syndrome symptoms are intensified, showing more negative emotions and reducing their motivation. When people's thermal sensation vote is −0.13, the best performance can be obtained. But the changes in human performance are not only caused by objective environmental factors, but also by psychological factors such as emotion and motivation. When people's negative emotions decrease or their motivations increase, performance will also increase.