Background Most metro system are underground, airtight, with inadequate ventilation and massive gatherings, posing health risks to metro riders. Objective To evaluate air quality of Metro Line 1 in a city, and provide suggestions and basis for preventing harmful factors and protecting the health of passengers. Methods Station halls, station platforms, and metro carriages of Metro Line 1 in a city were monitored in summer (from July to August in 2021) and winter (from January to February in 2022). Six metro stations were selected by stratified sampling. Each station and carriage were monitored for three consecutive days in rush hours (9:00–11:00 and 19:00–21:00) and non-rush hours (11:00–13:00), with the same monitoring frequency. The monitored indicators were physical factors (temperature, relative humidity, wind speed, illumination, and noise), chemical factors (carbon monoxide, carbon dioxide, inhalable particles, formaldehyde, benzene, toluene, xylene, ammonia, and ozone), biological factor (airborne total bacterial count), and radiation factor (radon). The monitoring results were compared by location, time period, and season. Results According to the Hygienic indicators and limits for public places (GB 37488—2019), the selected physical factors did not meet the standard, especially the temperature and relative humidity of station hall and platform, and wind speed and noise of carriage. The results of physical factors varied significantly by location (P<0.05). In summer, the temperature of carriage [M (P₂₅, P₇₅), 23.9 (23.3, 24.6)℃] was the lowest, and the wind speed [0.78 (0.37, 1.11) m·s⁻¹] and noise [76.0 (72.0, 80.3) dB] of carriage were the highest; in winter, the temperatures of station hall and platform were the lowest [16.2 (13.2, 17.2)℃ and 16.2 (13.4, 17.0)℃, respectively], the relative humidity of carriage [26.4% (24.2%, 27.9%)] was the lowest, and the wind speed and noise of carriage were the highest [0.83 (0.47, 1.18) m·s⁻¹ and 74.5 (70.1, 78.3) dB, respectively]. The physical factors varied significantly by time period (P<0.05). In summer, the temperature was the lowest during morning rush hours [24.0 (23.0, 24.8)℃] and non-rush hours [24.2 (23.2, 24.9)℃], and the relative humidity during evening rush hours was the lowest [41.9% (37.0%, 47.8%)]; in winter, the temperature was the lowest during morning and evening rush hours [16.8 (13.4, 19.7)℃ and 16.5 (15.1, 19.4)℃, respectively], the relative humidity during the non-rush period was the lowest [26.8% (24.7%, 28.6%)], and the wind speed during evening rush hours was the highest [0.28 (0.19, 0.51) m·s⁻¹]. All measured chemical factors, biological factor, and radiation factor met the national standard (GB 37488—2019). Conclusion The chemical, biological, and radiative factors are complied with the national standard (GB 37488—2019) except physical factors such as temperature, relative humidity, wind speed, and noise. We suggest that the metro operators make full use of air conditioning system in combination with humidifiers to better regulate temperature and relative humidity, and .arrange working hours reasonably and provide noise-proof earplugs for carriage staff to protect against noise hazard.