Abstract: An electromagnetic transient equivalent modeling method for input series output parallel（ISOP） type DC-DC converters composed of dual active bridge（DAB） unit was proposed. The existing power electronic transformer（PET） often includes the ISOP type DAB converter as an intermediate link, but due to the complexity of the DAB unit and the ISOP structure, the efficiency of the electromagnetic transient accurate simulation is extremely low. To this end, an equivalent modeling method for the ISOP type DC-DC converter constituted by DAB was proposed. Specifically, the components of the DAB unit were discretized using the trapezoidal integral method and the transformer was decoupled through analyzing. Then both the DAB unit and the series-parallel node were eliminated by the nested fast solution method, so that the entire DAB converter was equivalent to a Thevenin（Norton） equivalent circuit containing only four external nodes. After one step simulation, the internal node information, such as the output current of each DAB unit, can be updated. The accuracy and speed up factor of the proposed model were verified in PSCAD/EMTDC environment. The results show that the equivalent modeling method can accurately simulate the steady state and transient process of the system, and can greatly improve the efficiency of electromagnetic transient simulation.