With the access of increased renewable energy sources, the conventional AC distribution network is hard to flexibly adjust the line voltage and power flow. Therefore, the soft normally open point(SNOP) is applied to increase the flexibility of AC distribution network. The existing converters for SNOP use many submodules(SMs) and passive components, which have the poor economy. On this basis, a multiport AC-AC-DC converter for SNOP with one medium-frequency transformer(MFT) is proposed in this paper. The proposed multiport AC-AC-DC converter is based on the back-to-back(BTB) cascaded H-bridge (CHB) converter structure, and uses several LC resonant circuits and one MFT to replace the multiple DC-DC converters in the existing BTB CHB converters. Therefore, many SMs and passive components can be saved. Moreover, the voltages at multiple AC and DC ports of proposed multiport AC-AC-DC converter are completely decoupled, and the multiple AC and DC systems can be adjusted independently. The detailed circuit structure, control principle and design procedure of multiport AC-AC-DC converter are introduced in this paper. Finally, the simulation and experimental results verify the effectiveness of multiport AC-AC-DC converter.