This paper presents intensity calculation of the cylinder block of the underwater instrument cabin which has outer diameter of 220mm, thickness of 7.5mm, and length of 1660mm. With simplifying force of the cylinder block to the problem of elastic plane stress, its’ internal stress is calculated under internal pressure and external pressure, in the case of the simulation tests and working state. Calculations and simulation indicate that: its’ maximum internal stress exceeds the elastic yield limit at 30MPa, and the cylinder block cannot work in 3000 meters underwater. At the same time, it proves that the maximum internal stress first starts from the inwall of the cylinder block with the method of using internal pressurization test, which is consistent with the working state, but because the maximum internal stress in the test state is less than the maximum internal stress in the working state, the test method may misjudge. Finally, through calculation, it is specified that the theoretical work water depth of the cylinder block is 1300 m and the axial deformation is about 0.5 mm, radial deformation is about 0.1mm, which provides reference for the seal design.