The invention relates to the technical field of constant-current driving power supplies, and particularly relates to a fixed-frequency constant-current BUCK circuit. The fixed-frequency constant-current BUCK circuit is characterized in that a sampling circuit is arranged at the output end of a power supply terminal, voltage at both ends of the sampling circuit is relatively stable, the sampling circuit is connected with an LED circuit in series at the same time, acquisition current of the sampling circuit is equal to driving current of the LED circuit, a control circuit judges the state of the driving current of the LED circuit according to the acquisition current, a control signal is outputted when the driving current of the LED circuit stays at an unstable state so as to control the duty ratio of the PWM waveform, the operating state of a switching circuit can be changed by changing the duty ratio of the PWM waveform, and thus the driving current of the LED circuit is adjusted. By adopting the technical scheme provided by the invention, the acquisition current is stable, thus the driving current of the LED circuit is enabled to be relatively stable, and the service life of the LED circuit is prolonged.