Abstract: To fully exploit the flexible regulation capabilities of solid state transformers (SSTs) in the microgrids, an autonomous power mutual support control for an AC/DC microgrid system interconnected by a multi-terminal SST is proposed in this paper. According to the independent active power and frequency or active power and DC voltage droop characteristics of the AC/DC sub-microgrids interconnected by the SST, the normalized instantaneous active power imbalance variables in each sub-microgrid are defined so that the extent of the power imbalance in the entire microgrid can be evaluated. On this basis, to equalize all the normalized variables of each sub-microgrid, a combined control strategy for the three-stage converters of the SST is designed, realizing the overall power imbalance sharing among the four sub-microgrids autonomously. A small signal model is established to optimize the key parameters of the proposed control. Controller hardware-in-loop experiments based on the RT-LAB verifies that the proposed scheme can autonomously and flexibly realize the power mutual support among the interconnected AC/DC sub-microgrids.