Abstract: To explore the resistance mechanism of low-hardness polyurea/steel composite structures (PSCSs) penetrated by high-velocity fragments, ballistic tests were carried out. Failure modes and ballistic resistances of PSCSs were analyzed, and were compared with those of monolithic steel plates. On the basis of stress waves, ballistic resistance mechanisms of PSCSs were probed. Results show that the front polyurea layers mainly exhibit shearing plugging under high-velocity penetration by fragments, while the failure modes of rear steel layers change from shearing plugging to petalling. Under the same total areal density, the whole ballistic resistance of PSCSs is lower than that of monolithic steel plates, but the ballistic resistance efficiency of rear steel layers is promoted due to the influence of front polyurea layers. The whole ballistic resistance of PSCSs first decreases and then increases, instead of constantly decreasing, as the areal density ratio of front polyurea layer to rear steel layer increases. For a PSCS, the rear steel layer is its main energy absorption component.