Some external pressurized thin walled cylindrical shells could only be stiffened by small stiffening rings due to structural limitations. Such stiffening rings can only partially stiffen the cylinder which is less effective than big stiffening rings.Eigenvalue buckling analysis was done for a series of external pressurized thin walled cylindrical shells with various stiffening rings,which demonstrates two different types of buckling mode according to stiffening ring dimensions. When the stiffening ring is strong enough,buckling occurs only in the cylindrical shell with wave number bigger than 3,and the critical external pressure is irrelevant to the dimension of the stiffening ring,which should be defined as strong stiffening structure,and the ring as strong stiffening ring. However,when the stiffening ring is small,buckling occurs in both the cylindrical shell and the stiffening ring with wave number changing from 3 to 2 as the stiffening ring gets stronger,and the critical external pressure increases with the dimension of the stiffening ring,which should be defined as weak stiffening structure,and the ring as small stiffening ring. By parametric study,the design boundary curve was worked out for the weak and strong stiffening structures. For the weak stiffening structure,the design boundary curves have been also worked out to separate the 2 waves and 3 waves buckling. Empirical formulae were given out for the weak stiffening structures. Compared with the allowable external pressure given by the approximate calculation of HG / T20582-2011,the result by the suggested method is about 100% ,450% higher,which is in favor of light weight design of structures.