Low phosphorus scale inhibitor was synthesized with hydrolyses polymaleic anhydride (HPMA), 1-hydroxy ethylidene-1, 1-diphosphonic acid (HEDP) and polyacrylic acid (PAA), and carboxyl-terminated hyperbranched polyester (CHBP) as the main components, and then, its scale inhibiting performance was evaluated by calcium carbonate deposition method and deposition test of copper tube. The molecular structure of CHBP was proved by FTIR. The morphology and crystal structure of scale samples was proved by SEM and XRD. The results shows that the optimum composition of low phosphorus scale inhibitor is the above 4 components with mass ratios of m(HEDP): m(CHBP): m(HPMA): m( PAA) = 1:9:80:10. The scale-inhibiting rate of the composed reagent reached 87.5% when the dosage was 15 mg/L. The carboxyl groups of scale inhibitor prevent the formation of calcium carbonate scale. The introduction of CHBP can obviously reduce the content of total phosphorus, and the changed morphology of scale sample could stable in the water and flow easily, resulting in scale could not grow and aggregate in the surface of the heat exchanger.