The invention provides a method for designing a servo-constrained tracking controller of an underactuated mechanical system. The method comprises a step of constructing a dynamic model of the underactuated mechanical system containing parameter uncertainties and effectively decomposing the uncertainties in the system, a step of viewing a tracking performance requirement to be realized by the system as a servo constraint and constructing a second-order form of the servo constraint, a step of designing an adaptive law robust controller based on the constructed dynamic model and a second-order servo constraint, a step of performing stability analysis on the constructed adaptive law robust controller, and a step of adjusting main parameters in the constructed adaptive law robust controller andanalyzing a control effect. According to the design of the servo-constrained tracking controller of the underactuated mechanical system, the influences of the system parameter uncertainties and external interference can be effectively processed, at the same time, the system is quick and stable, and a given tracking performance requirement is accurately tracked.