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This paper proposed an adaptive backstepping control scheme for the position tracking problem of ElectroHydrostatic Actuator (EHA) with unknown dead-zone nonlinearity, parameter uncertainties and mismatched disturbances. Different from previous results, the boundaries of uncertainties,nonlinearity and disturbances can be completely unknown. Moreover, some of the uncertain parameters are time-varying and non-differentiable. To eliminate the effect of nonlinearity, parameter uncertainties and disturbances caused by the change of load and environmental conditions, a series of quadratic compensated terms and adaptive update laws combined with modification factor are introduced. Through stability analysis, it is proved that the proposed method is globally stable and the position tracking error can converge to the arbitrarily small compact set. Simulation results demonstrate that the proposed control algorithm is effective.