In this paper, by optimizing synthesis process of α,ω-hydroxyalkyl telechelic polydimethylsiloxane, α,ω-bis(3-(1-methoxy-2-hydroxypropoxy)propyl)polydimethyl siloxane (PMTS), the yield of hydrosilylation product, 1,3-bis(glycidoxypropyl) tetramethyldisiloxane exceed 86.5%. By tracing the change of methanol (gravimetry) and measuring the change of molecular weights of polydimethylsiloxanes at different reaction time (titration), the optimum reaction time of methoxylation reaction and ring-opening polymerization was determined as 8 and 12 h. Using α,ω-bis(3-(1-methoxy-2-hydroxypropoxy)propyl)polydimethyl siloxane with different molecular weights, waterborne polyurethane-polydimethylsiloxane block copolymer were prepared. The influences of molecular weights and content of α,ω-hydroxyalkyl telechelic polydimethylsiloxane on the waterborne polyurethane-polydimethylsiloxane block copolymers were investigated in detail. The addition of α,ω-hydroxyalkyl telechelic polydimethylsiloxane could improve the water-resistance property obviously and increase the elongation at break. However, the mechanical property was reduced with increase of content and molecular weight of α,ω-hydroxyalkyl telechelic polydimethylsiloxane.