Abstract: To address the issues of large volume, high cost, and slow equalization speed of existing active equalization schemes in long-series battery pack applications, this paper proposes an active equalization method based on an isolated multi-port converter. This equalization scheme relies on a high-frequency multi-winding transformer and a multi-port structure, enabling simultaneous equalization and electrical isolation of multiple battery cells using only a single converter. Taking a three-active-bridge converter with three ports as an example, this paper elaborates on its equalization working principle and power transfer characteristics. To overcome the challenges of port coupling and asymmetric transformer leakage inductance, a single-phase-shift power decoupling control strategy under specific constraints and an external tuned inductor hardware compensation scheme are designed. Finally, an experimental prototype of an equalization system comprising 2 battery modules is built. Theoretical and experimental results show that the proposed scheme offers advantages such as easy scalability, compact structure, controllable equalization current, and high efficiency, providing an effective solution for high-performance, low-cost equalization of long-series battery strings.