The wasting of the superheated steam or heating the oil reservoir not sufficiently might be caused by the uncertainty of steam injection period during the steam soak, therefore, and a two-dimensional, unsteady state mineshaft-stratum-oil reservoir mathematical model was established, and the temperature fields of the stratum and the oil reservoir were analyzed. Furthermore, the influences of steam injection period to the heat affected radius during the steam soak were discussed. The results show that the heat affected radius of the oil reservoir increases with steam injection period duration and reaches the maximum when the steaming for 5 d, then nearly no longer changed after that. The heat in the oil reservoir began to transfer to the stratum when the steaming for 3 d, and the temperature fields of the stratum and the oil reservoir are nearly the same for 6 d. The steam injection period should be controlled in 4~5 d during the steam soak.