The invention discloses a composite current constraint control method of a direct-current buck converter. The method comprises: a state space averaging model of a direct-current buck converter nominal system is established by using a capacitor voltage and an inductor current of the direct-current buck converter as state quantities; a reference current constraint controller is established; according to parameter perturbation of the direct-current buck converter system, input voltage fluctuation and load sudden-change disturbance, a disturbed-state averaging model of the direct-current buck converter is established; a generalized proportional-integral observer is constructed and a time-varying disturbance estimation value is obtained; the reference current constraint controller is introduced to carry out compensation on time-varying disturbance to obtain a composite current constraint controller; and calculation is carried out to obtain a controlled quantity, the controlled quantity is outputted by an output module to obtain a PWM drive signal, and a switching tube of the buck converter is controlled, thereby realizing voltage control of the buck converter. Therefore, requirements of high rapidness and accuracy of output voltage tracking and the anti-disturbance performance of the system can be met while the current constraint function is met.