The existing mathematical model of magneto-rheological damper (MRD) could not describe its characteristics correctly, so dynamic characteristic test on large scaled MRD was carried out. According to the test data, the dynamic nonlinear feature on elastic, damping and current saturation characteristics of MRD was analyzed with variable excitation displacement, excitation velocity and controlling current. Bouc-Wen model was not accurate enough to reflect hysteretic loop in low speed and lack hysteretic characteristics in high speed. It was also unable to express the offset phenomenon of damping force caused by pressure accumulator in single rod damper. Thus, a modified Bouc-Wen model was established. The parameters of the new model were identified with the test data, and the slope and width of hysteretic loop in velocityforce curves under high and low speed emphatically analyzed. The modified Bouc-Wen model was verified by experiment data finally.