Hydrogenated diamond film exhibits a high surface conductivity, which is very suitable for many in-plane micro-electronic and microelectrochemical devices. However, the surface conductivity mechanism of hydrogenated diamond film remains unclear up to now. It inevitably retards its further applications. This work is to elucidate the effects of active adsorbate and water molecule on surface conductivity of hydrogenated diamond film. By the first principles method based on density functional theory, several models corresponding to hydrogenated and oxygenated diamond (100) surfaces physisorbed with various active adsorbates are built up. The adsorbed species include H3O+ ion mixed with H2O molecules with different concentrations. The adsorption energy, equilibrium geometry and density of states corresponding to the adsorption system are investigated. At the same time, the electron populations for different atoms of the physisorbed adsorbates are studied. The results show that the equilibrium geometry of H3O+ ion relaxes signifi-cantly after adsorption on hydrogenated diamond (100) surface. In addition, its adsorption energy increases dramatically compared with the system of individual H2O molecule adsorbed on hydrogenated diamond (100) surface. It follows that the strong interactions occur between H3O+ ion and hydrogenated diamond surface. With the concentration of the adsorbed H2O molecules increasing, the adsorption energy between the adsorbate and hydrogenated diamond (100) sur-face decreases gradually. It indicates that the interactions between H3O+ ion and the substrate weaken as the water molecule concentration increases. Concerning the electronic structure of H3O+ ion adsorbed on hydrogenated diamond (100) surface, shallow acceptors appear near Fermi level, which arises from charge transfer from hydrogenated diamond surface to adsorbed H3O+ ion. Therefore, hydrogenated diamond surface exhibits a p-type conductivity. With regard to the mixed adsorptions of H3O+ ion and H2O molecule, no significant effect on its conductivity is detected, though its surface energy band structure changes. At the same time, the electron transfers from hydrogenated diamond (100) surfaces to the adsorbates are also similar for all the systems with the adsorbates including one H3O+ ion and different H2O molecules. Thus, the adsorbed H2O molecule concentration in this work has no effect on the surface conductivity of hydrogenated diamond surface. However, the adsorbates containing H2O molecules and H3O+ ion physisorbed on oxygenated diamond (100) surfaces do not exist stably. The H3O+ ion will decompose into one H2O molecule and one H atom, which form H—O bond with one O atom of oxygenated diamond surface. All the oxygenated diamond surfaces with various adsorbates exhibit an electric insulativity.%采用基于密度泛函理论的第一性原理方法，构建了不同活性质吸附氢修饰和氧修饰金刚石(100)表面，计算了氢修饰和氧修饰金刚石(100)表面吸附体系的平衡态几何构型和态密度。结果表明，氢修饰金刚石表面与H3 O+离子间具有较强的相互作用，在费米能级附近出现浅受主能级，电荷会发生从氢修饰金刚石表面向吸附H3 O+离子迁移，从而呈现p型导电性；当吸附物为H3 O+离子和H2 O分子混合吸附时，能带结构发生改变，但是其导电性并没有发生变化。相比之下，含水分子和H3O+离子的吸附物在氧修饰金刚石表面将发生分解，不能稳定存在，吸附体系仍呈现绝缘性质。
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