The invention discloses a method and system for detecting the air gap of a wheel speed sensor. The method comprises the following steps: firstly calibrating the air gap mark bit of the wheel speed sensor, wherein the calibration process comprises: providing a gear ring and a Hall sensor; after the Hall sensor is powered on, adjusting the distance between the gear ring and the Hall sensor so that the Hall sensor outputs a square wave signal having a high level and a low level when the gear ring rotates; increasing a resistance value between the output end of the Hall sensor and the ground in order that the Hall sensor outputs a square wave signal having a high level, a medium level and a low level when the gear ring rotates; selecting one, that is consistent with the size of the air gap of the wheel speed sensor, from the positions of a plurality of medium levels as the air gap mark bit of the wheel speed sensor; and during the operation of the wheel speed sensor, detecting the air gap of the wheel speed sensor according to the level of the air gap mark bit of the wheel speed sensor. The air gap between the Hall sensor and the gear ring in the wheel speed sensor can be automatically detected by the method and system.