In multi-input-multi-output relay systems with amplify-and-forward protocols, the source-relay and the relay-destination links can be treated as two groups of spatial channels. In this letter, an adaptive spatial channel mapping matrix is presented to be used between the two groups. A unified framework of the spatial channel mapping matrix design is presented, which includes various relay protocols. The spatial channel matrix is designed to reduce the relaying noise power or the local noise in the destination node. Simulation results demonstrate the advantage of the adaptive spatial channel mapping on the system reliability and capacity with different relay protocols.