The structure of water molecules encapsulated in single-walled carbon nanotubes (SWCNTs) was studied using a self-consistent charge density functional tight binding method with dispersion correction. The most interesting and important feature observed is the diameter shrinkage of SWCNTs when water chains are confined inside them. The diameter shrinking of SWCNTs may be due to the van der Waals and H-π interaction between water chains and SWCNTs. The binding energy decreases with the increase of the nanotube radius. But when the radius is increased to 6.78 ?, the binding energy is a little increased, and the water chain has changed as a "book-like" structure, which suggests that the weak hydrogen bonding in the isolated water chains is larger than the interaction between water chains and the SWCNTs.