In order to solve the problem of vertical interconnection of inter-layer signals in three-dimensional integrated circuits, based on the excellent characteristics of thick film substrates, an ultra-wideband coaxial to stripline to microstrip transition structure is designed, which can work up to 80 GHz. This vertical structure uses LTCC as the substrate and spin-coats the BCB film for system-in-package. The structure utilizes a “water droplets” matching method and is embedded in the air cavity to suppress parasitic capacitance, which effectively improves the RF transmission performance of the interconnect structure. The simulation results show that the back-to-back (coaxial-stripline-microstrip-stripline-coaxial) structure has a return loss of less than -17 dB in the frequency range of 0~80 GHz, and the insertion loss is better than -0.5 dB, and VSWR are lower than 1.35, which has good ultra-wideband transmission characteristics.