Abstract: This paper has studied in detail the elastic-plastic solutions of the stress and displacement of cylindrically orthotropic composite pressure vessel under the action of an internal pressure p (with Tsai-Hill Yield Criterion). The formulas of the yield pressure, the limit load and the region of shakedown have been given. According to the Elastic Loss-Effect Criterion, whole Yield Design Criterion and the Explosive Loss-Effect Criterion adopted internationally at present, Comparisons are made with the isotropic solutions. Numerical calculation shows that not only will the yield pressure of the pressure vessel made with fiber-reinforced composite materials increase greatly (the larger the value of k the larger the value of increase percentage), but also the resistance to deformation will increase when the ratio between the inner and outer radii k is equal to or greater than 0.79.