The invention provides a synthesis method for preparing alternating organic silicon polymers at a low temperature.The method includes the steps that under the protection of nitrogen, polycondensation is conducted on diphenyl silanediol, 1,4-bis(hydroxydimethylsilyl)benzene, 4,4'-bis(dimethylhydroxysilyl)diphenyl ether and ureido silane with different side groups in chlorobenzene at a low temperature, then deposition is conducted with methyl alcohol, deposits are put in a vacuum dryer to be dried to obtain the alternating organic silicon polymers of different structures.The method has the advantages in that reaction conditions are mild, the prepared polymers are completely-alternating linear high-molecular-weight organic silicon polymers, and the structures of the polymers can be changed by changing the side groups of ureido silane.