The ¹H NMR speotra of mixtures of dichlorodimethylsilane,octamethyloyclotetrasilogane and α,ω-diohloropolydimethylsiloxane with various n values were measured and a quantitative relationship was derived.Our results indicated that ¹H NMR method can be used to investigate the reaction between dichlorodimethylsilane and dimethyloyolopolysiloganes.The reaction between dichlorodimethylsilane and octamethyloyolotetrasiloxano with different Cl/Si ratios (R) and under different temperature has been studied.It has been found that n values can be controlled and the equilibrium constants at 220℃ and 200℃ for the interaction of two end groups giving middle group plus a diohlor odimethylsilane molecule arc 0.11 and 0.10 respectively.The mechanism of the above roaotion was proposed.