Abstract: Nonagonal modular multilevel converter is a multi-port and direct AC/AC converter with the advantages of low frequency and high reliability, which has attracted much attention in the field of fractional frequency transmission of offshore wind power. At present, the research on nonagonal converter usually does not decouple the internal electric quantity but directly controls it, and the single control methods commonly used have some problems, the control effect needs to be improved. To improve its dynamic response and anti-interference ability, this paper combines passivity-based control with sliding mode control on the basis of establishing decoupling model of nonagonal converter. Finally, the PBC-SMC control system simulation of nonagonal converter are built on Matlab, and the experimental results under different various steady, dynamic, and transient working conditions verify the feasibility of the PBC-SMC control. Compared with PI and PBC strategy, PBC-SMC strategy has faster response speed and higher power quality. In addition, the effectiveness of the proposed PBC-SMC strategy is verified on the RT-LAB experimental platform.