The invention discloses a composite additive for turbine oil. The composite additive comprises base oil, an anti-oxidant, a rusting inhibitor, a foam inhibitor, a metal deactivator, an extreme pressure additive, a demulsification agent, copper oxide powder and magnesium metal powder. The anti-oxidant is a mixture of diarylamine and alkylphenol. The rusting inhibitor is an alkylsuccinic acid derivative. The foam inhibitor is polydimethylsiloxane. The metal deactivator is triazole. The extreme pressure additive comprises alkyl phosphate ester and salt. The demulsification agent is polyalkoxypolyamine. The composite additive comprises, by weight, 12-14% of the anti-oxidant, 9-11% of the rusting inhibitor, 7-9% of the foam inhibitor, 10-12% of the metal deactivator, 6-8% of the extreme pressure additive, 11-13% of the demulsification agent, 1-2% of the copper oxide powder, 1-2% of the magnesium metal powder and the balance the base oil. The composite additive is prepared by the simple processes, has a price lower than those of the same types of other products, has high quality, effectively and comprehensively improves performances of the turbine oil, prolongs a use period of the turbine oil, improves turbine oil extreme pressure resistance, comprehensively improves oxidation resistance, corrosion resistance, foam resistance and oily sludge resistance of the turbine oil, is biodegradable and completely exceeds environmental requirements on the same type of products.