The invention discloses a bituminization rubber particle modifying agent and a preparation method thereof. The bituminization rubber particle modifying agent comprises the following compositions in parts by weight: 20-30 parts of petroleum asphalt, 10-15 parts of rock asphalt, 40-60 parts of discarded tire rubber powder, 2-5 parts of plasticizer and 3-6 parts of polythene. According to the invention, the hard petroleum asphalt is taken as a liquid transitional medium, and the rock asphalt is used for filling and degrading the softened discarded rubber tire powder, so that waste rubber powder can be subjected to high-temperature degradation in a double screw machine; in addition, the plasticizer is used for softening waste rubber powder to enable the molecular structure of rubber powder to be comparatively incompact, and then the rock asphalt is filled; the prepared modifying agent has the advantage of improving the high-and-low temperature performance of asphalt, so that the performance grade of the asphalt modified by the modifying agent can reach PG 88-28, the performance of the modified asphalt is far superior to other polymer modified asphalt, and the modified asphalt product cannot be isolated, cannot deposit, and can be stored at low temperature or high temperature for a long time; the bituminization rubber particle modifying agent has excellent compatibility with asphalt, the normal technical index of the asphalt is superior to the traditional rubber asphalt, the rubber modified asphalt manufacturing technology and modifying equipment can be greatly simplified, the modified asphalt production efficiency can be improved, and the production cost can be reduced.