A method for detecting a thickness of an optical disc is provided. Firstly, a beam intensity signal and a focusing error signal are generated. Then, a first time interval for a focus point of the laser beam to move from a first layer to a second layer of the optical disc is acquired according to the beam intensity signal or the focusing error signal. Then, a second time interval between two peak values of an S curve of the focusing error signal is detected. Afterwards, the thickness between the first layer and the second layer is calculated according to a known S-curve detection range, the first time interval and the second time interval. The S-curve detection range is multiplied by said first time interval and divided by said second time interval to obtain the thickness between the first layer and the second layer of the optical disc.