The present invention provides a control loop bandwidth adjusting system for adjusting a control loop bandwidth of an optical reproducing device. The optical reproducing device comprises a data signal processing circuit, which uses the control loop bandwidth to process the data signal. The control loop bandwidth adjusting system comprises a control loop bandwidth generator for generating the control loop bandwidth and a control loop bandwidth according to whether a defect signal corresponding to the defect data is received or not. If the control loop bandwidth adjusting module receives the defect data entry signal, the defect data duration signal, and the defect data exit signal, the control loope bandwidth is adjusted to a first bandwidth value, a second bandwidth value, and a third bandwidth value respectively.