The experimental observation that H2O molecules could be dissociated with a unity sticking coefficient on Si(100)-2x1 surface in low temperatures (especially as low as 80K) was not well understood since earlier theoretical attempts showed a relative high dissociation barrier.In this paper, we find that the additional H2O molecule tends to cluster with the adsorbed one, with corresponding adsorption energy as great as 0.88 eV in the most stable configuration according to our density-functional theory calculations.It is 2.8 times larger than that of interaction between two H2O molecules due to the H-bond interaction.Moreover, there are remarkable charge transfer happening between cluster and substrate.It indicates that the Si(100) substrate has played a crucial role in the process of water adsorption.Especially, the calculated barriers of two possible dissociation paths are nearly lowered by an order of magnitude with significant impact from the H-bond interaction between the H2O molecules, leading to a feasible explanation of the H2O molecule dissociated at low temperatures on the Si(100) surface.