The effect of lanthanum-cerium mischmetal on the kinetics and the microstructure of the super-cooled austenite continuous cooling transformation in 20MnCrNi2Mo wear-resistant cast steel by L78 RITA quenching thermal dilatometer, QUANTA-400 scanning electron microscope, JEM-2100 transmission electron microscope and so on. The ICP-MS inductively coupled plasma mass spectrometry was used to measure the amount of rare earth solid solution and the JEM-2100 transmission electron microscope was utilized to probe the existential state of the solid-solution rare earth. The action mechanism of the solid-solution rare earth was analyzed combined with the measurement of the interface diffusion coefficient of La. The results show that the CCT curves are moved to the bottom right, the hardenability is improved and the twin substructure in lath martensite is increased due to the effect of the rare earth. The trace amount of solid-solution rare earth can exist in grain boundary, dislocation and other crystal defects. They can reduce the grain boundary energy, block the diffusion channels and then defer the nucleation and growth of the new phase. So the phase transformation and the microstructure are influenced.