The invention relates to a phosphorus-free compound water treatment agent with excellent corrosion inhibition property and a preparation method thereof, belonging to the technical field of chemical products. The invention is used for solving the problem that the existing water treatment agent contains phosphorus, can not be easily biodegraded, and has poor corrosion inhibition property and the like. The water treatment agent provided by the invention is prepared from the following components in parts by mass: 1-4 parts of aconitic acid-acrylic acid copolymer, 0.5-2 parts of sodium polyaspartate and 0.25-1 part of zinc salt. The preparation method provided by the invention comprises the following steps: copolymerizing aconitic acid, which is generated by dehydration of citric acid, and acrylic acid; and compounding the copolymer with sodium polyaspartate and zinc salt to obtain the phosphorus-free compound water treatment agent. The water treatment agent provided by the invention is free of phosphorus and has favorable scale inhibition property; and the synergistic action of the three compounded substances ensures the high corrosion inhibition property in water of complex quality. The phosphorus-free compound water treatment agent is biodegradable, belongs to environment-friendly products, and has the advantages of low use amount, low carbon and environmental protection in use.