A power detector for detecting an output power of a power amplifier is provided. The power amplifier is used to transmit signal power to the antenna. The power detector includes a phase shifter, a multiplier and a calibration unit. The phase shifter sequentially applies a plurality of candidate phase calibration amounts to the output voltage of the power amplifier to generate a plurality of adjusted signals with different phases in a calibration mode. The multiplier sequentially mixes the replica current and each of the plurality of adjusted signals, to generate a plurality of analog signals with different phases. The calibration circuit determines a target phase calibration amount of the phase shifter from the candidate phase calibration amounts according to the plurality of analog signals.