The phase-covariant quantum cloning has an important application in quantum cryptography. By exploiting a general unitary transformation, the fidelity of clones is derived. The result shows that there exists a class of optimal phase-covariant quantum cloning. For these explicit transformations of the phase-covariant quantum cloning, the fidelity of clones is maximal and equal. Based on the cavity quantum electrodynamics, a scheme to implement the transformations of quantum cloning is proposed.