Providing comfortable indoor thermal conditions in educational buildings is essential for students' learning performance and general satisfaction. This study investigates seasonal variations in thermal environment and comfort in classrooms in the cold area in China through year-long field surveys, which involve concurrent indoor thermal conditions measurement and students' subjective responses surveys. Overall, 1973 university students were surveyed. Distinct seasonal differences were observed in room temperature, relative humidity, and CO₂ concentration, resulting in significant differences in neutral temperatures for each season. The neutral temperatures in summer and winter are 26.2 °C and 22.4 °C, respectively. The 80% of the acceptable temperature range was 21.3–25.4 °C in winter, and the upper acceptable temperature limit in summer was 29.6 °C. Windows and doors were opened less during cold conditions with low indoor and outdoor temperatures, which resulted in the indoor CO₂ concentration level increased to 2500 ppm. Compared to the PMV, the scissors difference exists in non-air-conditions environments in spring, summer, and autumn but can accurately predict the central heating environments in winter. And the adaptive thermal comfort method fits better with the subjects' thermal comfort. Overall, it is suggested that for classrooms in cold areas in China, cooling is needed in summer when room temperature exceeds 30 °C, while proper ventilation shall be provided in winter.