The environmental instability of single‐ or few‐layer black phosphorus (BP) has become a major hurdle for BP‐based devices. The degradation mechanism remains unclear and finding ways to protect BP from degradation is still highly challenging. Based on ab initio electronic structure calculations and molecular dynamics simulations, a three‐step picture on the ambient degradation of BP is provided: generation of superoxide under light, dissociation of the superoxide, and eventual breakdown under the action of water. The well‐matched band gap and band‐edge positions for the redox potential accelerates the degradation of thinner BP. Furthermore, it was found that the formation of P‐O‐P bonds can greatly stabilize the BP framework. A possible protection strategy using a fully oxidized BP layer as the native capping is thus proposed. Such a fully oxidization layer can resist corrosion from water and leave the BP underneath intact with simultaneous high hole mobility. Protected by native oxide: A three‐step picture of the ambient degradation of black phosphorus (BP) is given. A possible protection strategy using a fully oxidized BP layer as the capping is proposed. Such a fully oxidized layer can resist corrosion from water and leave the BP underneath intact with simultaneous high hole mobility.