A power adjusting circuit, an LED power supply and an LED luminaire. The power adjusting circuit includes: a zero-crossing detecting unit, adapted to generate a zero-crossing detection signal according to an high-frequency signal, wherein the zero-crossing detection signal is a pulse wave or a square wave, and the high-frequency signal is a sine wave or an AC wave; a signal processing unit, adapted to determine a second number of pulses or square waves of the zero-crossing detection signal corresponding to an adjustment signal that is input externally, according to a first number of pulses or square waves of the zero-crossing detection signal corresponding to a full-load operation of a functional device; a counting unit, adapted to generate a switch control signal according to the first number and the second number; and a switch unit, adapted to control an output of the high-frequency signal according to the switch control signal.