A quantum key distribution scheme based on single-photon polarization encoding was presented. In the protocol,Alice first sends string photons to Bob at random states of four single-photon states. Bob then does a unitary operation on it,sends it back to Alice and then a single photon is measured by Alice at last. The quantum key distribution is accomplished. The protocol needs a noiseless subspaces or subsystems. And the advantage is that it only needs single-photon states,local operations and polarization measurement. The security is garanteed theoretically.