To obtain the influence of outer environmental condition on the indoor airflow distribution for the 25T air-conditioned train in Qinghai-Tibet, the Realizable k-ε model is adopted to numerically analyze the indoor temperature and velocity fields of the running train. The results indicate that solar radiation has great effect on the indoor temperature field and it is meaningful for improving the thermal comfort to adjust the air supply parameters with the outer environmental condition. Under different air supply conditions, it is the conjugating relation between the indoor temperature field and velocity field that leads to different thermal comfort in different regions of the running train. In addition, the operating modes exchange under different outdoor environmental conditions is also discussed and some additional functions of the using air-conditioning system are put forward too.