This paper studies the mechanism of microbeads on the CRTS III self-compacting concrete using test methods including the concrete expansion,slump,T500,cement paste viscosity,etc.The results show that the proper amount of microbeads can significantly improve the workability of concrete.When the content of microbeads is 1.5%,the workability of concrete is the best,T500 decreases from 14 s to 5 s,and BJ decreases from 30 mm to 17 mm.The results of cement paste viscosity test and concrete workability test show that the action mechanism of microbeads is to reduce the value of the expansion time T500 by reducing the viscosity of concrete.However,microbeads can improve the yield stress of concrete,which is not conducive to the increase of concrete expansion.The research result has theoretically guiding significance for the preparation and material selection of CRTS III self-compacting concrete.