Abstract: The uranium solution critical facility is an overhead double vessel, which has complicated structure and huge unsymmetrical three-dimensional sizes, if the thermal stress analysis is carried through using three-dimensional entity model, the model has so many elements and the general computer has not enough solving ability. The element model must be simplified effectively and logically in order to finish the solution while the error is not too much. At last the structure which is influenced slightly by heat is omitted, only the zone which has higher temperature grads is selected, and the unsymmetrical restricting method is adopted to apply the boundary condition to the predigested model effectively. The solution model is simplified greatly while the reliability of the results is ensured, so the satisfying thermal stress results are gotten. It is very important for simplifying the solution model logically and applying the boundary conditions effectively according to the given structure and loads when carrying through the thermal analysis of the unsymmetrical structure with huge sizes.