The present invention provides a composite efficient low-phosphorus corrosion-inhibiting and scale-inhibiting agent for low hardness water, wherein the components comprise, by mass, 15-25% of a heptadecenyl hydroxyethyl imidazoline quaternary ammonium salt, 5-15% of a phosphono carboxylic acid copolymer, 10-20% of an acrylic acid-acrylate-sulfonate copolymer, 0.1-1.5% of benzotriazole, 5-15% of anhydrous zinc chloride, 1-3% of a co-solvent, and the balance of water. According to the present invention, the composite efficient low-phosphorus corrosion-inhibiting and scale-inhibiting agent is suitable for water with characteristics of low hardness, high alkalinity and strong corrosion, has alkalinity of less than 600 mg / L, and has excellent corrosion inhibition and excellent scale inhibition; and the total phosphorus content in the composite efficient low-phosphorus corrosion-inhibiting and scale-inhibiting agent of the present invention is lower than the total phosphorus content in the organic phosphates and the phosphate agents, the total phosphorus content in the industrial discharge water is less than or equal to 1.0 mg / L, and the composite efficient low-phosphorus corrosion-inhibiting and scale-inhibiting agent has characteristics of biodegradability, no pollution, environmental protection, low cost, easy compounding, good stability, less consumption, good synergy among various components, stable performance, excellent corrosion inhibition, and excellent scale inhibition.