The various weapons and equipment mounted on a warship must be secured for the safe transport. A cargo lift system with multi-stage watertight door is installed on the warship for such safe transport. The multi-stage watertight door of cargo lift should be able to transport heavy cargo with the guarantee of the watertight to facilitate the maritime military operations. In this study, structural strength evaluation and reliability analysis of the multi-stage watertight door of cargo lift were performed considering load conditions based on the MIL-STD load test specification. For the evaluation of structural strength, the three-dimensional finite element model of multi-stage watertight door of cargo lift was created and finite element analysis was performed by applying the load conditions. Reliability analysis was performed using Monte Carlo simulation(MCS) in order to improve design safety by considering uncertainty of operating environment. In the reliability analysis, the reliability level of structural strength results considering an operation load variation were evaluated.