The invention relates to modified guar gum and a preparation method thereof, relates to the field of vegetable gum modification and provides the high-yield modified guar gum without high-temperature and strong base conditions and the preparation method of the modified guar gum to overcome defects in the prior art. According to the method, the guar gum is added to a mixed solution of organic matters and water, a polymerizable functional monomer and an initiator are added to the mixed solution, the mixture has a heterogeneous reaction, and the modified guar gum is obtained; the functional monomer comprises N-vinyl-2-pyrrolidone and N-alkyl sulfonated acrylamide; a characteristic functional group includes but is not limited to sulfonyl and a heterocyclic group. The method is mild in condition and doesn't need alkalization, and a reaction is not required to be performed in a nitrogen atmosphere. The yield of the modified guar gum can reach 100%; the modified guar gum has efficient viscosity increasing and temperature resistance, the viscosity can reach 40 mP*s when the mass concentration is 0.1%, and the viscosity is kept higher than 70 mP*s when the mass concentration is 0.3% and the temperature ranges from 20 DEG C to 80 DEG C.