〔Abstract〕 Objective A magnetic resonance nano-contrast agent containing trimanganese tetroxide was synthesized, its characterization was explored, and its contrast efficiency and safety performance were explored. Methods Polyacrylic acid(PAA) was used as a pH-responsive polymer to react with manganese sulfate to obtain nanoparticles. The molecular formula of the synthesized nanoparticles was determined by X-ray photoelectron spectroscopy and X-ray diffractometer. Transmission electron microscopy, particle size analyzer, inductance Coupled plasma emission spectroscopy and 7.0 T magnetic resonance imaging instruments were used to study the morphology, magnetic resonance properties, cell biocompatibility, and material stability of nanoparticles. Results Manganese oxide samples were obtained by the reaction of glutathione, polyacrylic acid and MnSO4. The sample was observed under transmission electron microscopy as petal-shaped nanoparticles with a diameter of about 100 nm. X-ray photoelectron spectroscopy and X-rays diffractometer results showed that its composition was Mn3O4; at pH 4.5, the 24 h Mn2+release was still less than 0.5%, which proved its good stability and safety; magnetic resonance imaging results showed that the pH was acidic under the conditions, the manganese oxide nanoparticles had extremely high contrast performance. When the pH was 4.5, the longitudinal relaxation rate reached 13.0 mmol-1·s-1, indicating that it could respond to the tumor microenvironment; CCK-8 showed that co-cultivation of the manganese oxide nanoparticles with gingival fibroblasts(HGF) for 24 h and 48 h did not produce cytotoxicity, proving that the material had good biocompatibility. Conclusion A new nano-contrast agent with trimanganese tetroxide was successfully synthesized, which has good biocompatibility and the highest contrast efficiency under acidic conditions, providing a basis for its further clinical research.