Abstract: According to new standard,the consumption of steel of purlin in engineering changes with the variation of loading.In the paper,based on The Codes Of Technique Of Door-type Steel Support Structure For Light Houses and The Standard Of Technique Of Cold-formed Thin-wall Bar Structure(GB50018-2002),genetic algorithm is used to make optimization design of size of section of the purlin under the general conditions of loading and distance between poles.The consumption of steel of new stand and traditional standard are analysed and compared.The result of paper has good value of application.