Abstract: The unsteady inverse heat conduction calculation is the key point for the fatigue monitoring system of a nuclear power plant and the focus of the article. Firstly, the linearity between inner and outer wall temperatures was studied which was the foundation of the application of the unit transient method. Secondly, a finite element model for one dimensional unsteady inverse heat conduction was built. Thirdly, the outer temperatiure was numerically calculated from inner wall temperature and vice versa. Finally, one group test data verified the method correctness. Known as prospect, the method described above will be a key contribution to the fatigue monitoring system.