Objective To study the stability, electronic structure and the UV absorption of ZnO by surface molecular absorption. Methods Based on first-principle calculations within density functional theory, we firstly optimized the structures of different ZnO systems. The systems included O2, H2O molecules adsorbed onto the pure ZnO films and the ZnO films with oxygen vacancies. Then, we calculated the electronic structure and optical properties of different ZnO monolayers. Results By calculation of adsorption energy, we found that molecular adsorption was beneficial for improvement of stability of the systems. And the most obvious was oxygen molecular absorption. From the energy band and density of states figures, we found that molecular adsorption benefited the electron transition between valence band and conduction band, which could improve optical absorption. The figure of charge density difference indicated there was interaction between surface molecules because of charge transition between ZnO and surface molecules. Optical absorption figure displayed the improvement of optical absorption. By comparison, we suggested that water molecular adsorption on ZnO monolayer with oxygen vacancies was the best system because it had the stronger UV absorption, higher carrier concentration, smaller carrier effective mass and better stability. Conclusion The oxygen vacancies in the ZnO monolayer and the oxygen or water vapor in experimental environment influenced the UV response of ZnO. We hope our calculations could provide reference for the study on UV detector of ZnO films.