The invention belongs to the technical field of surface topography control of nanometer-scale. The invention relates to a preparation method for magnetorheological fluids based on control on cobalt-nickel alloy topography by carbon quantum dots, especially to a preparation method for magnetorheological fluid based on surface roughness controllable cobalt-nickel alloy. Carbon quantum dots are used as a surfactant to control the magnetorheological property of the materials from the point of changing the surface roughness and wetting property. The method mainly comprises the following steps of preparing carbon quantum dots, preparing a cobalt-nickel alloy dispersion solution, preparing cobalt-nickel alloy modified by the carbon quantum dots, and mixing the above cobalt-nickel alloy with silicone oil. The preparation process is simple, controllable, low-cost, environmentally friendly, clean, non-toxic and suitable for mass production. The shear stress yield of the prepared magnetorheological fluid is high and the application environment is friendly.