The present invention discloses a Trellis-Coded-Modulation (TCM) decoder applied in a receiver, wherein the TCM decoder includes a branch metric unit, a path metric unit, a trace-back length selection circuit and a survival path management circuit. In operations of the TCM decoder, the branch metric unit is configured to receive multiple input codes to generate multiple sets of branch information. The path metric unit is configured to calculate multiple survival paths according to the multiple sets of branch information. The trace-back length selection circuit is configured to select a trace-back length, wherein the trace-back length is determined according to a signal quality of the receiver. The survival path management circuit is configured to return the multiple survival paths for the trace-back length in order to generate an output code.