A channel estimation method for an OFDM relaying system, which is based on an EM algorithm and applied to an AF OFDM relaying system, wherein channels from a source node to a relaying system and from the relaying system to the destination node in a wireless communication system are respectively estimated at the destination node. The channel estimation method of the present invention comprises steps of: constructing a system model; setting an expectation function; performing maximization processing; and performing iteration. The method of the present invention respectively estimates the channels from the source node to the relaying system and from the relaying system to the destination node, whereby are optimally combined the signals of the direct path (source-to-destination) and the signals of the relay path (source-to-relay and relay-to-destination) at the destination node.