Abstract: In order to investigate the ultimate strength of imperfect composite cylindrical shell under hydrostatic pressure, a moderate thick filament wounded glass fiber-reinforced polymer (GFRP) composite cylindrical model was fabricated, and its initial deflection was measured before hydrostatic external pressure test. Then the compo-site cylinder model was tested up to failure under hydrostatic external pressure, the ultimate bearing capacity, strain response and failure mode were analyzed comprehensively. Based on the measured initial deflection and failure mode, a nonlinear finite element model was established, which the initial geometric defects and in-plane damage of composites during the loading procedure were simultaneously considered in the numerical model. Through programming interface subroutine with ABAQUS software, the failure process and mechanism of moderate thick GFRP cylinder model were obtained, and numerical result were compared and comparative verified with the experimental results. The results show that the ultimate load of moderate thick GFRP cylinder model is almost increasing before model collapse, and the final failure mode is the compression failure of the composite material, while the global buckling failure mode is not obvious. After considering the geometrical defects and damage evolution of composites, the ultimate strength of the moderate thick composite cylindrical shell is agreed well with the experimental result. It can be used as a method to predict the ultimate strength of medium thickness composite cylindrical shells with defects. On this basis, the key parameters affecting the ultimate strength of medium thickness composite cylindrical shells were studied, which can provide reference for the design of deep-sea composite pressure shells.