The invention provides a method for preparing PMMA (polymethyl methacrylate)/CNTs (carbon nanotubes) composite materials. The method includes: preparing surface-modified carbon nanotubes from carbon nanotubes and auxiliaries such as organosilicon compounds, polycarboxylate, polyoxyethylene ether and acrylic resin under certain conditions by a physical blending process; adding the surface-modified carbon nanotubes and initiators into methyl methacrylate matrixes; uniformly dispersing the carbon nanotubes, the initiators and the methyl methacrylate matrixes to obtain mixtures and polymerizing the mixtures by means of standing under low-temperature conditions to obtain the PMMA/CNTs composite materials. The method has the advantages that monomer MMA (methyl methacrylate) is polymerized in a standing state under the effect of ultrasonic waves and the joint effects of the carbon nanotubes (CNTs) and the initiators at the low polymerization temperatures of 15-80 DEG C; reaction conditions are mild, surface chemical modification by the aid of strong oxidizing agents such as strong acid can be omitted, polymerization reaction can be carried out at the low temperatures, the method is low in energy consumption, technological conditions are simple, and operation is easy to implement; self-acceleration and implosion phenomena can be prevented in reaction procedures, the reaction procedures are stable and mild, the surfaces of products obtained by the aid of the method are smooth and flat, the carbon nanotubes are excellently dispersed, and the glass-transition temperatures of the obtained PMMA is increased.