In this paper, we have investigated the geometries and electronic structures of B-N co-doped carbon nanotube with inside adsorption of water molecules. The charge distributions, band structures and local density of states are calculated by using the first-principles theory in detail. The results show that the water molecules can adsorb stably on the inside surface of the doped nanotube with slight deformation. The π and π* subbands shift upward depending on the sites of the adoptions. The investigations will be beneficial to the biological application of B-N co-doped nanotube.