Abstract: The higher-order coherence function theory has been applied to study the fifth-order nonlinear effect on polarization beats of a cascade three-level system(FPBCTMS) in Markovian field.Based on analysis of the cases that pump beams are chaos fields,phase-diffusing field and real Gaussian field,compassed of beat signal in FPBCTMS are obtained.The effects of amplitude fluctuation and phase diffusing in the measurement of the energy-level difference are discussed.