An automatic calibration method suitable for an electronic apparatus comprising a proximity sensor is disclosed. The automatic calibration method includes steps of: resetting data comprising a minimal index and a maximal index when a specific function is activated; updating the minimal index and the maximal index according to readings from the proximity sensor; resetting the minimal index when the proximity sensor detects a far event according to a threshold value of the proximity sensor; resetting the maximal index when the proximity sensor detects a near event according to the threshold value; and, re-calculating the threshold value according to the minimal index or the maximal index every time the minimal index or the maximal index is updated.