Poly aspartic acid or its salts is a kind of newly innocuous, environmental friendly biodegradable polymer, recognized as green material, and widely applied in such areas as agriculture, medicine, commodity, water treatment. In order to prepare new poly aspartic acid-metronidazole (PASP-MTI) nanoprodrug, observe its trichomonacidal effect in vitro, and explore its probable mechanism, the nanoprodrug PASP-MTI was synthesized by DL-aspartic acid and metronidazole in a relatively mild condition. Its structural characteristics were attested by means of infrared spectrum (IR), ¹H nuclear magnetic resonance (¹H-NMR), transmission electron microscope(TEM) etc measurements. The release behavior of metronidazole in PASP-MTI nanoprodrug was investigated by the dialysis method. The MTT assay was employed to test PASP-MTI’s trichomonacidal effect in vitro and the apoptotic-like processes of trichomonas were detected by fluorescence microscope and flow cytometer (FCM). The IR and ¹H-NMR showed that the metronidazole was conjugated to poly aspartic acid high molecular chains by ester bonds. Synthesized PASP-MTI nanoprodrug was globular and had a mean diameter 404.8 nm with favorable dispersity. The drug loading was about 30% and the cumulative release profile was delayed (about 38.01% of MTI loading in the PASP-MTI nanoprodrug was released within the first 24 h and 86.64% in 30 d, while the release of free metronidazole was almost complete by 24 h). The trichomonacidal activity of PASP-MTI nanoprodrug increased remarkably{IC₅₀ of PASP-MTI and free metronidazole was 1.22 mg/L and 2.85 mg/L, respectively}. Observed by fluorescence microscope, the nuclei of trichomonas presented a series of changes similar to apoptosis such as chromatin agglutination, nuclear fragmentation, etc. The FCM study indicated that the apoptotic rate in the PASP-MTI group was the highest of all groups, up to 35.69%. It is concluded that PASP can be effectively used in drug delivery systems, the synthetic novel poly aspartic acid-metronidazole nanoprodrug has higher drug loading, better the role of prolonged-release and higher trichomonacidal effect in vitro. Its mechanism may be related with enhancing the polymer prodrug’s internalization and inducing the apoptosis of trichomonas.