Abstract: The limit loading formulae of thick-walled tubes under the action of combined inner pressure with axial force, are derived based on the twin shear unified strength theory. The strength differential effect in tension and compression is considered and the curve of limit loadings is plotted. In these formulae, the results given by Tresca criterion, linear approximate Mises criterion and twin shear stress yielded criterion can be obtained respectively when different coefficients are chosen. Whether the whole thick-walled tube has entered into the yield state can be decided based on the curve. Limit loading curves of thick-walled tubes under different yield criteria are plotted so that differences among them can be compared.