A pre-decoded tail-biting convolutional code (TBCC) decoder and a decoding method thereof are provided. The decoder includes a pre-decoder, a storage module, and a control module. The pre-decoder receives a current state, a neighboring state, and a current path status corresponding to sequential data encoded in TBCC, generates predicted decoded bits, and determines whether states corresponding to minimum path metrics of neighboring stages are in continuity according to the current state, the neighboring state, and a current path status. The storage module is connected to the pre-decoder and stores the predicted decoded bits. The control module is connected to the storage module and the pre-decoder. In addition, the control module selects to output the decoded bits from the storage module when the continuity between the states corresponding to the minimum path metrics of the neighboring stages reaches a truncation length.