A system and a method for calibrating an ambient light sensor (ALS) are disclosed. The ALS, an adjustable resistor and a switch are located on a first surface of a printed circuit board (PCB), and the adjustable resistor and the switch are connected in series between an adjustable probe of the ALS and the ground. A resistor is connected between two pads located on a second surface of the PCB via two probes touching the pads. A controller connected to the PCB reads a light sensitivity of the ALS and calculates a calculated resistance value of the adjustable resistor by a formula “detected light sensitivity/resistance value of the resistor=objective light sensitivity/resistance value of the adjustable resistor”, wherein the objective light sensitivity and the resistance value of the resistor are given.