Disclosed is a polishing fluid for improving surfaces formed by fused deposition molding with ABS, consisting of 30%-40% by weight of polymethyl methacrylate and 60%-70% by weight of a mixture. This application further discloses a method of preparing the polishing fluid by mixing under heating. The polishing fluid provided herein can quickly form a film on surfaces of the workpiece to be processed and fill depressions of the surfaces of the workpiece while dissolving the protrusions on the surfaces to render the surfaces smooth and even.