The invention discloses a three-phase transformer electromagnetic transient low-and-medium frequency modeling method. The method comprises the steps of selecting iron core joints as magnetic circuit division points of a transformer model, and partitioning a transformer; building a magnetic circuit equivalent model according to magnetic conductive features of materials inside the transformer of allpartitioned areas and the spatial distribution of all parts of the transformer; conducting classifying according to physical properties represented by all magnetic resistance elements, establishing acircuit model according to relevant parameters and electromagnetic duality principles of the elements and conducting simplifying according to a topological connection relation; adopting a dynamic loss model according to loss characteristics of magnetic conductive materials of the transformer to simulate the loss of a transformer iron core; combining the circuit model and the dynamic loss model, wherein the combined model is used for simulating nonlinear excitation and loss features of the transformer iron core; calculating the parameters of the circuit part of the transformer, and adopting alinear transformer model for simulation; connecting the combined model for representing the transformer iron core transient characteristics to the low-voltage side of the linear transformer model; calculating the parameters of the transformer.