In order to study the failure mechanism of large warship broadside protective liquid cabin, the scaled model of warhead and two kinds of liquid cabin structure, including exposure model and airtight model, were designed based on the bearing characteristics of the liquid cabin. The combined damage experiment of high-velocity fragment and shock wave under warhead close explosion, including two kinds of posture, were performed. According to the damage of liquid cabin model after test, the failure mechanism of front plate and back plate of liquid cabin under typical load was analyzed, and the failure mode and mechanism of liquid cabin structure was summarized and analyzed. The result shows that the high-velocity fragment is the main defensive object of protective liquid cabin, the drag and cavitation of fragment are the main stage of liquid cabin deformation and fracture, and the shock wave and cavitation squeezing pressure caused by penetration of fragment cluster into liquid cabin structure are the major load, which make structure deform and damage.