A capacitive fingerprint sensor comprises a fingerprint capacitor, a reference capacitor, a first transistor, a second transistor, a comparator and a multiplexer. The fingerprint capacitor has a capacitance that is either a valley capacitance CFV or a ridge capacitance CFR, wherein CFV is smaller than CFR. One end of the reference capacitor CS is connected to the fingerprint capacitor, and the other end is connected to a trigger signal, wherein the trigger signal is initiated only during a precharge phase. The first transistor is configured to control the fingerprint capacitor during a scan line period. The second transistor is configured to precharge the fingerprint and reference capacitors. One end of the comparator is connected to the second transistor. The multiplexer is connected to another input end of the comparator for providing a first voltage VA and a threshold voltage Vth.