Abstract: In modern wireless communication system, differential circuit has stronger anti-interference ability than the single-ended circuit, so it has been widely concerned by scholars at home and abroad. In this paper, mixed mode S-parameters are introduced firstly, and then microstrip line and slotline, which are commonly used in differential device designs, and their mutual transformation are introduced. Two types of fully differential bandpass filters based on microstrip line and slotline conversion structures are presented in this paper. The two filters realize the differential-mode transmission characteristics of multi-band and broadband respectively, and the broadband common-mode suppression characteristic. Finally, the designs of various differential devices such as differential coupler, differential power divider and differential antenna are also given. The simulation results agree well with the measured results, which verify the correctness of the design method.