In this study, the structure that a stripline crosses with a narrow slot on the adjacent ground plane was rigorously studied in both a signal integrity and an electromagnetic interference point of view. A partial-element equivalent-circuit (PEEC) based circuit model was proposed to analyze the local region close to the slot, and it can be connected to the plane model and the transmission line model later. Based on an equivalent principal, the problem under a study can be converted to a mixed coupling problem between electrical surface current at the stripline and magnetic surface current on the slot. Therefore, the structure under the study can be treated as a complete cavity surrounded by the perfect electrical conductor at the top and bottom surfaces and a perfect magnetic conductor at four sides, so that the cavity Green's functions can be used in the PEEC formulation. The results obtained by the proposed method matched well with those obtained by a finite-element-based commercial tool.