The paper adopts the rate equation method to analyze laser cooling process of ytterbium（Yb） atoms.By dynamic behaviour of energy level evolution of Yb atoms

we succeed obtaining the cooling cycle transition of Yb atoms

i.e.singlet transition 1s0→1p1 and singlet to triplet transition 1s0→3p1

which is in good accordance to the experimental result.Neglecting the relative collisions

the method analyzes the mature Yb atomic system theoretically.And it can help trying to find a universal method to get cooling-cycle transition in complex atomic system to guide experiment and push forward the progress of ultra-cold atomic physics.