Disclosed are a method and apparatus for detecting frame boundary for a data stream received at an Ethernet FEC layer, as well as a decoding method and system for the same. The apparatus for detecting frame boundary may comprise: a buffer for buffering data in a data stream, a length of the data in the buffer being greater than one frame; a syndrome generator for calculating a current syndrome based on a first data item, a second data item, and an intermediate calculation result of a previous syndrome, wherein the first data item is the last bit in a current candidate frame, and the second data item is a bit preceding the current candidate frame; and a comparator for using the current syndrome to check whether the bit preceding the current candidate frame is a frame boundary of an Ethernet FEC layer. The apparatus for detecting frame boundary can improve the speed of frame boundary detection.