The invention discloses a tire self-mending agent and a method for preparing the tire self-mending agent. The tire self-mending agent comprises, by mass, 30-40% of deionized water, 0.1-1% of polyvinyl alcohol, 0.5-1% of cellulose ether, 1-5% of plant fiber, 0.1-1% of nanometer inorganic filler, 40-50% of alcohol anti-freezing agent, 1-5% of rubber particle, 0.5-2% of calcined gypsum, 0.1-1% of sodium nitrite and 5-10% of latex. A plug and a leakage-proof film formed by the self-mending agent after the self-mending agent is dried can seal a crack permanently, and the tire self-mending agent is good in air tightness, prevents a tire from leaking air, can endure heavy loads, and endures impact with the road surface. The tire self-mending agent is convenient to use, nontoxic, free of extraneous odor and strong in mending capacity and can be used for mending 6.8mm cracks in an inner tube , 10mm cracks in a vacuum tire of a motorcycle, 18mm cracks in a vacuum tire of an automobile and 38mm cracks in a vacuum tire of an engineering machinery vehicle.