The invention discloses an ultra-high molecular weight modified polysiloxane and a preparation method thereof. The ultra-high molecular weight modified polysiloxane is prepared by introducing sulfur element into polymethylvinylsiloxane or polymethylhydrosiloxane, wherein the sulfur element is introduced in the forms of thiazoles, dithiocarbamates and other groups, so that the reflectivity of polysiloxane is improved. On the other hand, the invention discloses the preparation method of the ultra-high molecular weight modified polysiloxane, comprising the following steps of putting the polymethylvinylsiloxane or polymethylhydrosiloxane into a reaction bottle, adding 0.5% to 1.5% of initiator, and stirring; then, adding the thiazoles or the dithiocarbamates, heating to 64 to 100 DEG C, and performing refluxing reaction for 5 to 20h, so as to obtain the ultra-high molecular weight modified polysiloxane. The preparation method has the advantages that the yield rate is high, and the industrialization is easily realized.