To control thermal cracks of concrete of thick arch lining of tunnel of big pan, based on basic assumption, the formula calculating thermal stress of arbitrary position and the maximal stress in arch lining restricted by surrounding rock is deduced. Moreover, the formula of thermal stress of arbitrary position, the maximal thermal stress is also presented, when the thick of arch lining varies according to the law of cosine. The change of temperature of concrete of arch lining in practical engineering is monitored. The maximal thermal stress in arch crown surface and the thermal stress in surface along the axis of arch are calculated using the formula deduced above is presented and is contrasted with its real strength. Conclusions show that the thermal stress in arch crown is the maximal stress, concrete lining do not crack under the condition of thermal stress less than its strength.