Abstract: The forms of water vapor in the atomspheric circumstance were observed using a triple quadrupole mass spectromter. There was not any single H₂O, which were detected, and H^＋(H₂O)_n , n＝3-37 were the main forms of water in gas phase. The collisional induced dissociation (CID) mass spectrometry was studied, which showed that the main dissociation products of H^＋(H₂O)_n (n＝4-16) were H^＋(H₂O)₄ and/or H^＋(H₂O)₅.