Using the theory of thin-walled members,this paper derived the total potential for bending and torsional buckling of C purlins under wind suction,the expression of the critical moments of the purlins was proposed,whose results were compared with those of ANSYS model. The critical moment formula without taking account of deformation effect before buckling was given,and the possible use of the model and the formulas were pointed out by comparing EC3 method with the test results,showing that EC3 method was conservative,yet the proposed formula was simple, whose calculated length coefficient was 10% smaller and bearing capacity was about 20% higher than EC3 method.