An image sensor driving unit, comprising a signal generator and a controller, is provided. The charge-transfer channel transfers the signal charges at a speed according to the frequency of a transfer signal. The signal generator transmits the first discharge signal and the second discharge signal to the image sensor. The first and second discharge signals are the transfer signals for the charge-transfer channel to carry out rapid discharge. The frequency of the second discharge signal is greater than that of the first discharge signal. The controller orders the signal generator to generate the first discharge signal during an overlap period when a driving period is at least partially overlapped with a discharge period. The controller orders the signal generator to generate the second discharge signal when the driving period is not overlapping with the discharge period.