In order to strictly solve the problem of persistent current in one-dimensional mesoscopic metal rings, a model with a perfect ring coupled to an exteal reservoir by a four-lead junction is introduced to independently describe in sequential tunnelling. Since the S-matrix is unitary, the current conservation keeps automatically. Since the quadruplet of the matrix problem can be reduced into two topologically separate doublets, it gives a simple and elegant method compared vith the former description with a 3×3 matrix.