Abstract: Considering the shortcomings of theories proposed previously about inflatable structures, an analytical relation between a wrinkling angle and a bending load is derived from a more reasonable wrinkling criterion that wrinkles are going to occur while the minor principal stress approaches to zero in an Euler-Bernoulli beam model. It is of importance to apply the relation to calculate its critical wrinkled load and collapse load. Further more, the range of the wrinkled region also can be sketched and used to predict the wrinkling tendency. Based on the computed example, we can draw some distinct conclusions: the collapse takes place at the odd point of the relation curve between the wrinkling angle and bending moment before the wrinkled region completely propagates around the circumference of the root cross-section; the projection of the sketched critical wrinkled boundary in the symmetrical plane is a curve but not a straight line. These calculation results keep a good consistency with the experiments which have been carried out.