Dynamic shearing experiment on TC18 titanium alloy was carried out by using a split Hopkinson bar process. Refining mechanism of grains in the adiabatic shear band of TC18 titanium alloy deformed under the conditions of high strain-rate were investigated by means of OM, SEM and TEM. The results of TEM observation show that the grains in the adiabatic shear band of TC18 titanium alloy are refined due to the combined action of three mechanisms. The first mechanism is that dislocations were rapidly reproduced, moved and clogged at the obstacles such as grain boundary, second-phase particles, and so on; then, cracks were generated in the head of clogging of dislocation due to huge stress resulted from the clogging of dislocation; finally, smaller grains were formed due to breakdown of elongation grain. The second refining mechanism of grains, which is presented by us first, is inner-necking mode. The third one is dynamic recrystallization.