Abstract: To obtain the initial condition of pollutants profile for natural ventilation numerical simulation in winter, the Realizable k-ε turbulence model was adopted to numerically analyze the C ₇H ₈ concentration field in a civil residence in Lanzhou. The results indicate that the indoor C ₇H ₈ concentration which comes from the floor and emits in outer normal direction is affected significantly by the coupled heat transfer between the indoor and the outdoor environment. The average value of C ₇H ₈ concentration in the horizontal section of Z=1.10m is three times of the limited value of the indoor air quality standard while the average indoor temperature and the thermal comfort can satisfy the resident's need when the temperature of floor surface is kept at 27℃ and the emitting time is up to 19 hours. Considering the climate condition and the residents’ living habits in Lanzhou, the pollutant emitting time, 19 hours, is thought to be reasonable to decide the initial condition of pollutant concentration in natural ventilation simulation.