It is well known that in the process of thermal oxidation of silicon, there are P _b-type defects at amorphous silicon dioxide/silicon (a-SiO₂/Si) interface due to strain. These defects have a very important impact on the performance and reliability of semiconductor devices. In the process of passivation, hydrogen is usually used to inactivate P _b-type defects by the reaction P _b + H₂ → P _bH + H. At the same time, P _bH centers dissociate according to the chemical reaction P _b H → P _b +H. Therefore, it is of great significance to study the balance of the passivation and dissociation. In this work, the reaction mechanisms of passivation and dissociation of the P _b-type defects are investigated by first-principles calculations. The reaction rates of the passivation and dissociation are calculated by the climbing image-nudged elastic band (CI-NEB) method and harmonic transition state theory (HTST). By coupling the rate equations of the passivation and dissociation reactions, the equilibrium density ratio of the saturated interfacial dangling bonds and interfacial defects (P _b, P _b0, and P _b1) at different temperatures is calculated.