Aiming at the problem that there are unmodeled dynamics by theoretic modeling and it is also difficult to identify, a Pseudo-Hammerstein-Wiener model with cascade connection of a two-segment polynomial nonlinearity block, a time-invariant linear system, and a backlash nonlinear term was adopted to model the electro-hydraulic servo system of excavator arm. The key term separation principle was used to decompose the model into a linear-in-parameter format, and a refined recursive least square method supplemented with the estimation of internal variables was proposed to identify the decomposed parameters. Experiments demonstrated that the identified Pseudo-Hammerstein-Wiener model approximated the actual system well. Comparing with Hammerstein and ARX model, the error of the Pseudo-Hammerstein-Wiener was reduced by 29% and 68%, respectively. 