The invention discloses a high-CTI, high-heat-resistance, halogen-free and flame-retardant PC/ ABS composition and a preparation method thereof. The composition comprises the following raw materials in parts by weight: 65-90 parts of polycarbonate, 3-10 parts of ABS resin, 4-10 parts of self-made phosphosilicate polymer, 3-10 parts of modified porous material, 1-5 parts of thickening agent, 0.1-0.3 part of antioxygen, 0.3-0.7 part of anti-dripping agent, and 0.2-0.5 part of lubricating agent. According to the high-CTI, high-heat-resistance, halogen-free and flame-retardant PC/ ABS composition,the defects of low CTI, high dosage of phosphorus based flame retardant, low flame retarding performance and low thermal deformation temperature of a traditional halogen-free flame-retardant PC/ ABSresin are solved; the complex system of the self-made phosphosilicate polymer and the porous material is selected, and a continuous net shaped carbon layer structure is formed through an electric field and electrolyte, so that the resin is high in CTI, and high in thermal deformation temperature; the thermal deformation temperature of the prepared PC/ ABS composition is up to 113 DEG C, and the CTI is up to 450V.