An advanced process control (APC) method for controlling a width of a spacer in a semiconductor device includes: providing a semiconductor substrate; providing a target width of a gate; forming the gate on the semiconductor substrate, in which the gate has a measured width; depositing a dielectric layer covering the gate, in which the dielectric layer has a measured thickness; providing a target width of the spacer; determining a trim time of the dielectric layer based on the target width of the gate, the measured width of the gate, the target width of the spacer, and the measured thickness of the dielectric layer; and performing a trimming process on the dielectric layer for the determined trim time to form the spacer.