The traditional stress analysis method of ring-stiffened cylindrical shell under hydrostatic pressure is to simplify the cylindrical shell as a bending beam in bidirectional stress state. With the emergence of new design features such as large depth loads and large thickness in deep-sea pressure structure, the assumption of bidirectional stress state for cylindrical shell is clearly unreasonable. This paper proposed a new simplified mechanical model for ring-stiffened cylindrical shell, which transforming the stress problem of ring-stiffened cylindrical shell under hydrostatic pressure into the superposition of bending beam stress under axial force and plane annulus stress under radial force, and thereby a three dimensional stress analysis method of ring-stiffened cylindrical shell has been derived. The application example results showed that the calculation results of the three dimensional stress analysis method has been in good agreement with the finite element simulation results, and it has better calculation accuracy and safer compared with the traditional stress analysis method.