Abstract: A transformer model considering deep saturation of the iron core is essential to accurately simulate lowfrequency transients of transformers. The defect of conventional T and Γ models is: saturation levels of each part are same when the core goes into deep saturation, which does not conform to the actual situation. Although this problem is resolved in the conventional π model, no π model that characterizes physical processes of the core from saturation to deep saturation has been presented. Based on the principle that the core is excited into deep saturation by a DC source, an AC coupling signal is used to measure the data of the magnetization curve in the saturation region, and then the entire magnetization curve is obtained. A method to distribute the data of the curve to two branches of the π model was proposed. The non-linear behaviors of magnetizing inductances were described precisely, then an improved π model for single-phase two winding transformers considering deep saturation was established. Two transformers with different core materials were selected for research. Parameters were obtained by open-and short-circuit tests plus deep saturation tests. The improved π model and six models based on T, Γ and π models were established on the ATP/EMTP platform. Inrush currents and DC bias currents were used to validate the improved π model. Results indicate that the maximum error of this model is less than 5%, far lower than other models, which demonstrates its accuracy for simulating low-frequency transients such as inrush currents. And this model can provide a fundamental transformer model for EMTP-type electromagnetic transient programs.