[en] This paper mainly discusses the nonlinear effect induced by the quantum coherence of the ground state fine structure energy-levels in a cascade-type four-level system. Coupling field excites the optical transition from the excited state lower fine structure energy-level to both two ground state fine structure energy-levels, which leads to a new absorption peak. It is shown that the smaller population relaxation rate between two ground state fine structure energy-levels is, the more significant quantum coherence is. However, this quantum coherence can be weakened by increasing the Rabi frequency of coupling field. (authors)