Abstract: Considering the different tensile-compressive strength of materials and the effect of intermediate principal stress, the unified solutions of the elastic limit internal pressure and the plastic limit internal pressure of a double-layered thick-walled cylinder under internal pressure are deduced, based on twin shear unified strength theory and a bilinear hardening model. The influence of parameters on the unified solution of elastic and plastic limit internal pressure is analyzed. It is shown that:each unified solution increases with increase of the radius ratio and unified strength theory parameter, but decreases with increase of the tension-compression ratio. The elastic limit internal pressure is irrelevant to the strain-hardening modulus coefficient, but the plastic limit internal pressure increases with the increase of the strain-hardening modulus coefficient. The delamination radius has a significant influence on the elastic limit internal pressure, but has a slight influence on the plastic limit internal pressure. For the engineering applications, reasonable wall thickness should be chosen so as to bear a greater internal force on the basis of safety. The consideration of the intermediate principal stress effect and tension-compression ratio can make materials develop their full potential ability.