Generation system reliability of a single area will be improved by the supports from other areas' generation capacity through interconnection， however the current generation reliability calculation algorithms cannot take this into account due to the computation burden. An exponential analytic model is proposed in this paper based on the generation system reliability of interconnected areas. The effects of installed capacity of each area and of transfer capability of tie lines on the generation system reliability in each single area are modelled by establishing the matrices of influence distribution indices. The generation reliability is calculated as an exponential function of regional reserve capacity and tie lines transfer capability， which can effectively consider the interaction between different areas. Test results on a simple two-area system and the TH-RTS indicate that the precision and effectiveness of the model.