Polyaspartic acid (PASP)-based green scale inhibitor has great potential application in water treatment. Here, we first synthesized PASP in ionic liquid. Then, an effective PASP-based green scale inhibitor was synthesized by ring-opening graft modification of PASP with both aspartic acid (ASP) and monoethanolamine (MEA). Its chemical composition was characterized by gel chromatography (GPC), Fourier transform infrared spectroscopy (FTIR), and 1H nuclear magnetic resonance (¹H NMR). Scale inhibition efficiency was measured by static scale inhibition tests. The results showed that the new PASP-based scale inhibitor has high scale inhibition to both CaCO₃ and Ca₃(PO₄)₂. When the concentration was increased to 2 mg/L, the inhibition efficiency of the new PASP-based scale inhibitor was 99% for CaCO₃, while when the concentration was raised to only 4 mg/L, its inhibition efficiency increased to 100% for Ca₃(PO₄)₂. Scanning electronic microscopy (SEM) and X-ray diffraction (XRD) were used to analyze the changes of crystal structure for CaCO₃ and Ca₃(PO₄)₂ after adding the new PASP-based scale inhibitor. The crystal size of CaCO₃ and Ca₃(PO₄)₂ became smaller and the crystal form became amorphous after adding the modified PASPs compared with adding pure PASP. Moreover, the modified PASP showed good biodegradation performance.