This present invention discloses a light-driving system capable of providing an accurate calibration of signal measurement and a method for performing the same, including an automatic power control (APC) circuit which is pre-calibrated for a signal measurement process. By enlarging at least one measured pad of the APC circuit, multiple grounding paths are established via a plurality of probes of a test instrument. An impedance effect predicted on the contact between the probes and the pad is diminished greatly. A voltage value on the pad can be accurately measured. Thus, a reference voltage value input to a first input of a comparator of the APC circuit can be determined on a basis of a specific condition when a ramping voltage value input to a second input of the comparator is substantially equal to a sum of a predetermined reference voltage value and the voltage value of the pad.