The invention relates to a biodegradable lubricating oil. The biodegradable lubricating oil is composed of base oil, an antioxidant, a corrosion inhibitor, an antifoaming agent, a metal deactivator, a demulsifier, an anti-wear agent and copper oxide powder, wherein the base oil is composed of synthetic base oil poly(alpha-olefin), synthetic base oil synthetic fat and synthetic base oil polyether; the antioxidant is anyone or a mixture of diarylamine and sterically hindered phenol, wherein the weight ratio of diarylamine to sterically hindered phenol in the mixture is 1:1; the corrosion inhibitor is a salt formed through using fatty acid and amine; the antifoaming agent is polydimethylsiloxane; the metal deactivator is a toluyltriazole derivative; the demulsifier is polyalkoxyphenol; the anti-wear agent is sulfurated triphenyl phosphate; and the copper oxide powder is nanometer powder, and the particle granularity of the powder is 25-35nm. The biodegradable lubricating oil has the advantages of simple preparation method, scientific and environmentally-friendly preparation scheme, degradability, high work temperature range, good lubricating characteristic, long service life, good protection to parts, high comprehensive performances, reasonable cost, realization of above 90% biodegradation of waste oil, extremely low gas discharge, and obvious energy saving effect.