OBJECTIVE： To prepare argininate betulinic acid， and to investigate the effect of the proliferation of triple-negative human breast cancer cell MDA-MB-231. METHODS： By using argininate as the solubilization carrier， argininate betulinic acid was prepared by co-grinding equal molar ratio of betulinic acid and argininate. The argininate betulinic acid was characterized with powder X-ray diffractometry， infrared spectroscopy and differential scanning calorimetry. The solubility of betulinic acid and argininate betulinic acid were compared. MTT method was used to assay the effects of 15， 30， 60， 120 μg/mL betulinic acid， argininate betulinic acid and 5-FU on the proliferation of MDA-MB-231 cell. RESULTS： Prepared argininate betulinic acid was a new phase which was different from the physical mixing of argininate and betulinic acid， among which carboxyl group of betulinic acid and amino group of argininate formed as a salt， and the salt had no obvious melting peak. Betulinic acid was almost insoluble in water. The solubility of betulinic acid in argininate betulinic acid aqueous solution was 50.72 μg/mL. Compared with betulinic acid， the inhibitory rate of argininate betulinic acid on the growth of MDA-MB-231 cell was increased significantly （P＜0.05）， there was no statistical significance between its effect and 5-FU （P＞0.05）. CONCLUSIONS： Argininate betulinic acid with good solubility is prepared successfully， and can inhibit the proliferation of MDA-MB-231 cell.