Interface in one complex system represents the logical or physical relationship integrating the components of the complex system or the components with their environment. It plays a quite significant role to guarantee the components designed by the designers of different disciplines integrate correctly in order to achieve the multidisciplinary integration. This paper presents an interface knowledge base to capitalise the design data and rules related to the interfaces. A knowledge-based engineering (KBE) design methodology in which the interface knowledge base is implemented is then proposed to help the designers to improve the multidisciplinary integration during the detailed design phase of complex systems. This KBE design methodology is finally demonstrated by a case study based on a partial discharge (PD) detection system.