The research of the learning environment has become an important part of education, because indicating how the learning environment affects learning efficiency has great significance. This research focuses on the indoor environment in summer, and aims to figure out which environmental factors have substantial influences on learning efficiency. The indoor air data and questionnaire data were collected in summer, and the nonlinear relationship between environmental factors and learning efficiency was identified by applying the automatic data mining function of symbolic regression. A model of this relationship was also established. In this paper, three major conclusions were made: (1) In the summer indoor environment, the temperature and carbon dioxide concentration are primary influential factors, especially the temperature; (2) The optimal temperature of the indoor environment is about 25°C, and the lighting condition also has a certain effect on learning efficiency; (3) The higher the carbon dioxide concentration, the lower the learning efficiency.