Based on the theory of thinwalled structures, the stability of coupled bending and torsional vibration of thinwalled beam was researched. According to the variation method and energy principles in mechanic, the oscillatory differential equation of BernoulliEuler thinwalled beam was deduced. The effect of axial force on the structure was considered, and then the characteristic equation of critical force was built. Finally, the analytic solution of differential equation was deduced by using method of order reducing and sweep method of axial force modifying. Calculation examples show that the results are convergent and agree well with the results of finite element method, which prove the effectiveness of this method.