With the deepening interdependence between the power system and the natural gas system (NGS), the integrated power-gas system (IGPS) has been a vital part of the energy structure. On one hand, interdependence improves energy utilization. On the other hand, the two-way energy flow brings new challenges to the reliable operation of IPGS. Therefore, the integrated planning model of IPGS considering N-1 constraint is proposed. Firstly, a framework of IPGS is developed to describe the N-1 contingency both in the power system and NGS considering cascading effects. Then an integrated planning model is proposed considering N-1 constraint and cascading effects between the power system and NGS. The reduced disjunctive model (RDM) is utilized to obtain the optimal planning scheme and reduce calculation difficulty. Furthermore, to further verify the effectiveness of the proposed methods, the integrated planning model is performed on an integrated gas and power test system.