To prevent thermal cracks caused by temperature changes in superlength concrete structures or massive concrete structures, this work defined the thermal boundary conditions and analyzed the thermal stress of the structures.Based on the basic theory of thermal conduction, a method to define the environment temperature that the structure served in was described. The basement of an actual super-long concrete structure was taken as an example to illustrate the method. The thermal boundary conditions were determined. Then, general finite element analysis was performed to on ANSYS simulate the thermal field of the structure under two worst cases including both temperature rising and temperature reduction. The calculation and analysis results of thermal stress showed that: the thermal stress gradient distribution varied along the thickness on the concrete members of the super-long structure, and the thermal fields in the joints of the concrete members were complex; the thermal deformation tended to be larger in the members further away from the center of stiffness; the stress concentration was distinctly in the turners of wall, interactions of wall and plate, under parts of hole, and the stress decreased as distance increased from the parts mentioned above.