An apparatus for performing power detection includes a squarer module and a calibration module that is coupled to the squarer module, where the squarer module includes a first squarer and a second squarer. The first squarer is arranged to convert an input signal of the squarer module into a first squarer output signal. In addition, the second squarer is arranged to output a second squarer output signal while a predetermined voltage level is input into an input terminal of the second squarer. Additionally, the calibration module is arranged to compare a difference between the first squarer output signal and the second squarer output signal with a reference signal to generate a comparison signal, and to compensate the difference according to the comparison signal, so as to perform a calibration on the apparatus. A method for performing power detection is also provided, and can be performed by utilizing the apparatus.