The composite phosphate-free scale inhibiting corrosion inhibitor for treating circular cooling water consists of scale inhibitor and corrosion inhibitor. The scale inhibitor consists of one or several of PASP, PVA, oxidized starch, polyacrylic acid, acrylic acid / acrylate copolymer and acrylic acid / acrylate copolymer with sulfo radical. The corrosion inhibitor consists of one or several of sodium salt / potassium salt / ammonium salt of organic salt, sodium / potassium / ammonium borate, nitrous organic matter, soluble molybdenate, soluble tungstate, soluble nitrate, soluble nitrite and soluble zinc salt. The composite scale inhibiting corrosion inhibitor has excellent scale inhibiting and corrosion inhibiting performance, is environment friendly, and is especially the treatment of hard circulation water with high calcium and high alkali content.