The invention relates to a multifunctional cleaning agent for oil fields, a preparation method therefor and applications thereof. The cleaning agent is composed of 0.2%-1.0% of composite nonylphenol polyoxyethylene ether active agents, 0.2%-1.0% of composite quaternary ammonium salt positive ion active agent chlorides, 0%-1.0% of composite acid pickling corrosion inhibitors, 0%-1.0% of composite Fe ion chelating agents, 0%-10.0% of organic carboxylic ester, and the balance being water. The cleaning agent has good acidolysis solution removing effects on inorganic chemistry scales of CaCO3, Fe2O3 and FeS at the temperature of 60-80 DEG C, has dispersion and melting effects on oil stains, wax scales and heavy oil stains, has a killing effects on oil field fungi, has a peeling effect on sticky dirt, and is suitable for cleaning of devices of oil field production well shafts, gathering pipelines, oil and water separation settling tanks, flocculation settling tanks, oil storage tanks, sewage storage tanks and the like.