In this paper, we study the relative stable molecular structures of uranium-wate r vapor system- For the uranium atom were used the relativistic effective core p otential and contracted valued basis sets (6s5p2d4f)/[3s3p2d2f], and for oxygen and hydrogen atoms were used 6-311G** basis sets- We obtain the elect ronic state, geometric structure, energy, harmonic frequency, mechanical property, et c-，of these six quintuple relative stable structures- It indicated that the first step of uranium-water vapor reaction is the interaction of uranium and oxyg en atoms- The thermodynamical stability of UOH2(5A1) was calculated an d its disassociation Gibbs free energy ΔG°decreased with increasing temper ature -The result showed that low temperature favoured its existence-