The invention discloses a preparation method of modified expanded polystyrene. The preparation method of the modified expanded polystyrene comprises the following steps: adding di(dioctyl phosphite) tetraisopropyl titanate into acetone, uniformly stirring, then adding carbon nano tubes for carrying out ultrasonic dispersion, and drying to obtain a nucleating agent; adding water, barium stearate, activated calcium phosphate, nekal, a defoaming agent, an antioxidant, a flame retardant and styrene into a reaction container, uniformly stirring, then sequentially adding dibenzoyl peroxide, tert-butyl peroxybenzoate, dicumyl peroxide and the nucleating agent, uniformly stirring, heating up to the temperature of 90 DEG C, and carrying out heat preservation for 3-4 hours, so that polystyrene is obtained; adding liquid butadiene-acrylonitrile rubber and melamine formaldehyde resin into a reaction container containing polystyrene, heating up to the temperature of 100-110 DEG C, uniformly stirring, adding a foaming agent, and carrying out heat preservation for 3-4 hours, wherein pressure in a heat preservation process is 1-1.2MPa; and cooling to room temperature, and filtering, so that the modified expanded polystyrene is obtained.