With the widespread application of lithium iron phosphate batteries, the production capacity of the yellow phosphorus industry has increased sharply, and the treatment of the highly toxic by‐product PH3 is facing severe challenges. In this study, a 3D copper‐based catalyst (3DCuO/C) that can efficiently decompose PH3 at low temperatures and low oxygen concentrations is synthesized. The PH3 capacity is up to 181.41 mg g−1, which is superior to that previously reported in the literature. Further studies indicated that the special 3D structure of 3DCuO/C induces oxygen vacancies on the surface of CuO, which is beneficial to the activation of O2, and then promotes the adsorption and dissociation of PH3. The doping of P after dissociation determines the formation of Cu‐P, and the eventual conversion to Cu3P leads to the deactivation of CuO active sites. More strikingly, due to the appearance of Cu3P, the deactivated De‐3DCuO/C (Cu3P/C) exhibited significant activity in the photocatalytic degradation of rhodamine B and photocatalytic oxidation of Hg0 (gas) and can also be a candidate as an anode material for Li batteries after modification, which will provide a more thorough and economical treatment scheme for deactivated catalysts.