The atomic - scale insights into the interaction of water with oxide surface are essential for elucidating the mechanism of physiochemical processes in various scientific and practical fields, since the water is ubiquitous and coats multifarious material surface under ambient conditions. By utilizing van der Waals density functional calculations, herein we report for the first time the energetically and dynamically favorable homolytic dissociative adsorption behavior of water over MgO(001) films deposited on metal substrate. The water adsorption on pristine MgO(001) is quite weak and the heterolytic dissociation is the only fragmentation pathway, which is highly endothermic with large activation barrier of 1.167 eV. The binding strengths for the molecular and dissociative adsorption configurations of water on MgO/Mo(001) are significantly larger than corresponding configurations on bare MgO(001). The homolytic dissociative adsorption energy of water on monolayer oxide is calculated to be - 1.231 eV, which is even larger than all the heterolytic dissociative adsorption states. The homolytic dissociative adsorption structure could be obtained by transformation reaction from heterolytic dissociative adsorption structure, presenting activation barriers 0.733, 1.103, 1.306, 1.571, and 1.849 eV for reactions on 1-5 ML films. With the increase of the oxide thickness, the homolytic dissociative adsorption energy decreases promptly, indicating the nanoscale of oxide film play a crucial role during water splitting. Ab initio molecular dynamics (AIMD) simulations were performed to confirm the stability of the ultrathin oxide films and the homolytic fragmentation configuration of water on the hybrid surface. Electronic properties are characterized to interpret the enhancement of chemical reactivity and properties of insulating oxide toward homolytic fragmentation of water.