Disclosed is a surface treatment agent for galvanized steel sheets, which comprises epoxy-containing silane derivates, alkyl-containing silane derivates, alkoxy silane derivates and one or more selected from the group consisting of a cationic polymer aqueous dispersion, a non-ionic polymer aqueous dispersion and a water-soluble polymer. Also disclosed is a method for producing it, which comprises mixing each component uniformly for 0.5 to 8 hours at 5 to 70° C. Further provided are galvanized steel sheets coated with the surface treatment agent on the surface and a production method thereof. The surface treatment agent for galvanized steel sheets has excellent corrosion resistance, alkali resistance, solvent resistance, recoating property, high temperature resistance and electrical conductivity.