Abstract: The inter-medial channel provides 3 kinds of the protection signals, such as the rod stop threshold, the reactor trip threshold and the anticipated transient without reactor trip protection signal. These protection signals are sprung by current, while in the safety analysis these protection signals are sprung by the true reactor power. The relationship between the currents of the inter-medial channel and the true reactor power may change. And if the relationship changed, the protection set point of the inter-medial channel current would drift. The factors which affect the relationship were analyzed and two kinds of the calibration methods were given in this paper. These two kinds of methods were also evaluated.