A mechanical ball-milling method for preparing a polydopamine-modified montmorillonite nanomaterial is disclosed. The method includes dispersing a montmorillonite material in an aqueous solution, stirring, concentrating and collecting a concentrated montmorillonite solution for use; adding dopamine hydrochloride to a buffer solution to prepare a dopamine hydrochloride solution, with a concentration of 0.2-1 g/mL, and adjusting the pH value of the dopamine hydrochloride solution; and adding the dopamine hydrochloride solution and the concentrated montmorillonite solution simultaneously into a ball mill jar to form a mixture, and then subjecting the mixture to a ball milling for 0.3-6 hours, pouring the mixture out of the ball mill jar, and subjecting to a solid-liquid separation by a centrifugation, and then washing a solid product with deionized water for 3-6 times, and removing water from the solid product, to obtain the polydopamine-modified montmorillonite nanomaterial.