It's essential to establish a dynamic model of a wheeled tractor with suspended driver seat including air spring and Magnetorheological (MR) damper to analyze its vibration characteristics and performance. The equivalent linearized model of the wheeled tractor always neglects the effect of nonlinear stiffness for scissors linkage driver seat and air spring with auxiliary chamber as well as dynamic characteristics of real tractor tyre and considers the driver seat as linear spring and damper elements, which causes the analysis to have a lower precision. We considered the effect of nonlinear stiffness for scissors linkage seat and air spring with auxiliary chamber as well as MR damper and dynamic characteristics of real tyre and developed a complete nonlinear dynamic model of wheeled tractor with suspended driver seat including air spring and MR damper. The experimental results demonstrate that the improved nonlinear dynamic model is more consistent with the actual state than the equivalent linearized model, and the validity of the proposed model is verified. Furthermore, the effect of forward speed, ratio of volume for air spring with respect to volume of auxiliary chamber, and area of orifice on the acceleration of wheeled tractor was also investigated, which can provide a theoretical basis for control algorithm design of semiactive vibration isolation system.