A human presence detector includes a microwave generator, a microwave receiver, a frequency mixing wave detector and a signal processor. The microwave generator is configured to emit and transmit a detecting microwave in a detection space. The microwave receiver is configured to receive a corresponding echo of the detecting microwave. The frequency mixing wave detector, linked to the microwave generator and the microwave receiver, is configured to perform a frequency mixing wave detection on the detecting microwave and the corresponding echo of the detecting microwave to output a primary detecting signal. The signal processor linked to the frequency mixing wave detector is configured to select a fluctuation signal at a predetermined frequency range in the primary detecting signal to amplify and output a secondary detecting signal. Accordingly, in response to the detection of the motion at the predetermined frequency range, a human (living) body is detected and determined in the detection space.