The invention designs an electromagnetic transient equivalent modeling method for an input-series and output-parallel cascaded power electronic transformer with double active bridges. The core technical scheme of the invention is as follows: 1, carrying out discretization equivalence on a switch group, a capacitor, an inductor and a primary/secondary side of a transformer to form a cascaded PET adjoint circuit; 2, listing a node admittance equation of a single H-bridge-DAB module, eliminating internal nodes by utilizing a rapid nesting simultaneous solving algorithm to obtain an equivalent circuit only containing four nodes, and meanwhile, reserving admittance information of the internal nodes; 3, performing equivalent simplification between PET modules to obtain an equivalent circuit onlycontaining four nodes of a plurality of H-bridge-DAB modules, and popularizing the equivalent circuit to three phases; 4, integrally solving by using electromagnetic transient simulation software toobtain equivalent four-terminal voltage and current of each phase; and 5, inversely solving and eliminating the voltage and current of the nodes, and updating equivalent parameters of the capacitor, the inductor and the transformer branch at the current moment.