In order to find out the optimal period of time of natural ventilation in winter, the realizable k-ε turbulent model of CFD is adopted to numerically analyze the pollutant concentration field in a residence in Lanzhou when the unsteady heat transfer through outerwalls is considered. The results indicate that the inlet temperature has significant effect on the indoor pollutants profile and migration, and different ventilation period corresponds to different duration under the same opening patterns. Considering the pollutants elimination efficiency and thermal comfort simultaneously, the ventilation beginning at 17:00 is the optimal ventilation mode.