A method for measuring propagation loss in a plane light guide plate, which includes the following steps of making an input light generated by an input light generator pass through an optical coupler, an optical reflection device individually or pass through the optical coupler first and then the optical reflection device, and finally propagate toward an optical measurement device to obtain at least one optical parameter according to the optical coupler, the optical reflection device or the combination thereof; and making the input light pass through the optical coupler and then the plane light guide plate, and further pass through the plane light guide plate from one side thereof toward the optical reflection device in order to reflect the input light toward the optical measurement device, so as to obtain a propagation loss of the plane light guide plate according to the at least one optical parameter.