The invention relates to the technical field of ultra-precision machining, and relates to a preparing device and a preparing method for composite particles for polishing. Magnetic particles coated with a binder on surfaces are input into a mixing dispersing mechanism through a first spraying mechanism, an abrasive material coated with the binder on surface is input into a mixing dispersing mechanism through a second spraying mechanism; mixed liquor which is obtained by sufficiently mixing the magnetic particles and the abrasive material in the mixing dispersing mechanism is sprayed to a conveying mechanism, the mixed liquor is quickly cured to form a composite particle cluster under action of a curing mechanism in a conveying process, and composite particle cluster is scraped off by a scraping mechanism at the tail end of the conveying mechanism; and the composite particle cluster is transferred into a ball mill mechanism for being ball-milled and stirred to form composite particle powder, and the composite particle powder is transferred into a screening mechanism to screen to obtain composite particles with magnetism within a set dimension range. According to the preparing devicedisclosed by the invention, the magnetic particles and the abrasive material are combined into the composite materials, so that the problem that polishing efficiency is low as abrasive material binding force is insufficient in a polishing process is solved.