By adding the time-varying gain into the backstepping method, this paper addresses the output tracking problem for strict feedback nonlinear systems that suffer unknown time-varying parameters and unmatched disturbances. First, a series of virtual controllers are constructed based on time-varying gain, and two types of adaptive laws are designed to compensate for time-varying parameters and disturbances. Second, a controller is constructed based on virtual controllers and adaptive laws. Benefiting from the introduced state transformation, the error of system output with tracking signal is proved to be convergence. Moreover, for matched disturbance problems, the designed controller ensures that system states and the error of system output with tracking signal converge to zero. Finally, the effectiveness of the proposed output tracking controller is verified by a second-order numerical example.