The present invention provides a method for cleansing a glass substrate of a TFT-LCD, includes the following steps: providing an ultrasonic cleansing machine, a glass substrate in-feeding conveyor, and a glass substrate out-feeding conveyor, wherein the ultrasonic cleansing machine includes a cleansing tank, a cleansing liquid contained in the cleansing tank, and first and second ultrasonic frequency generators arranged in the cleansing tank and having different frequencies; conveying a glass substrate with the glass substrate in-feeding conveyor into the cleansing tank; immersing the glass substrate in the cleansing liquid of the cleansing tank; generating ultrasonic waves of different frequencies with the first and second ultrasonic frequency generators to be applied to the cleansing liquid to effect ultrasonic cleansing of the glass substrate; and conveying the cleansed glass substrate out of the cleansing tank with the glass substrate out-feeding conveyor.