The present invention relates to a touch sensor with electrostatic immunity and sensing method thereof. The touch sensor includes a sensing electrode, a discharge element, a sensing-controlling terminal and an input-output (I/O) control terminal, wherein the discharge element is coupled between the sensing-controlling terminal and the I/O control terminal and the sensing electrode is coupled to the sensing-controlling terminal. The essence of the present invention is using charge-discharge through the sensing-controlling terminal and the I/O control terminal for sensing whether a conductor contacts the sensing electrode. There are corresponding compensations no matter an electrostatic charge on the sensing electrode is positive charge or negative charge. Therefore, the influence of electrostatic charge can be eliminated.