In order to analyze the principle of modification of asphalt modified by crumb rubber powder,by means of thermogravimetric analysis, infrared spectroscopy, SEM scanning, the effect of physicochemical reactions of 70^# asphalt mixed with different quantities (20%,25%,30%,35%,40%) on the mechanism of the swelling of modified asphalt after high temperature treatment was revealed from microscopic angle. Results show that:Rubber powder have good compatibility with asphalt and can be distributed evenly in asphalt. In addition to a small part of the rubber powder that has reacted chemically, most of the rubber, more than 50% of the rubber powder can exist as its original form; High temperature causes the rubber powder asphalt modified to generate new chemical matter by oxidation decomposition reaction. The swelling property and stability of modified asphalt will increase with the increase of the amount of rubber powder, but high content of the powder will affect the performance of modified asphalt, which can be improved remarkably by adding special oxidant.