The present invention relates to modified rubber powder and its preparation process and use. Structurally, the modified rubber powder has alkyl group, alkenyl group, hydroxyl group, carboxyl group, nitrile group or amino group in the side chain or has side chain connected via double bond reaction to butadiene, polystyrene, vinyl acetate, acrylate, acrylonitrile or other compound. The modified rubber powder has the structural features of rubber retained and thus excellent mechanical performance, physical and chemical performance and machining performance. The introduction of functional group to the side chain results in obviously raised compatibility with other rubber material, plastic and other polymer material, easy grafting reaction and functional group chemical reaction for further modification. In addition, the performances may be regulated via altering the sort and amount of the introduced functional groups.