A method for reducing a number of input terminals of an APC circuit is provided, where the APC circuit is arranged to control an optical pickup unit (OPU) within an optical storage device. The method includes: utilizing at least one switching module to ground one of a first and a second input terminals of the APC circuit at a time; and utilizing the switching module to electrically connect an APC front end within the APC circuit to a non-grounded input terminal of the first and the second input terminals, in order to receive a detection signal of a photo diode of the OPU through the non-grounded input terminal at the time. An associated APC circuit is also provided.