In this paper,a calculation model of Coldformed thinwall steel which under the axialcompression load was set up by using the coupled bendingtorsional deformation classical theory,and then the Lagrange function and the Lagrange equations were got;A Hamiltonian functions wasconstituted by introducing dual variables,the use of Hamiltonian mechanics leaded the question fromthe Lagrange system to the Hamilton dual system,then the Hamilton’s canonical equations whichwere used to analyze stability of thinwalled bar were established.The precise numerical value ofcritical load could be achieved according to the using of dichotomy theory and precise integrationmethod.This method has higher precision and the calculating process is simple and clean.This meth-od can be extended to apply to study the stability of the Variable Crosssection.