A pre-decoded tail-biting convolutional decoder and method are provided. The decoder includes a pre-decoder, a storage module and a control module. The pre-decoder receives the current state, the neighboring state and the current path corresponding to the sequence data encoded in tail-biting convolutional code (TBCC), generates predicted decoded bits, and determine whether the neighboring states corresponding to minimum branch metrics are continuous according to the current state, the neighboring state and the current path. The storage module stores the predicted decoded bits. The control module is connected to the pre-decoder and the storage module, and it outputs the predicted decoded bits from the storage module when the neighboring states corresponding to minimum branch metrics are continuity.