Abstract: The influence of signal integrity caused by the switch of reference planes in multilayer printed circuit boards was evaluated by the method of combining "field" and "circuit".Firstly, the path of return current in via transition structure has been analyzed, and the transient impedance between two reference planes was obtained by "field" method.Then the relationship between S parameter and node voltage based on equivalent circuit topological structure has been derived by equalizing via transition structure to two-port network.The computational formula of S parameter has been obtained.Finally, the return loss and insertion loss have been calculated with different via-radius and planes spacing.Research results show that the impedance of return current is decrease following the decreasing size between two reference planes.The return loss is lesser and the insertion loss is greater when the radius of via decreased, the impact of signal integrality caused by via transition structure was decreased.The computed result is accordant with the result of electromagnetic simulation.