Rectangular box section is widely used as columns in agricultural Greenhouse. Smaller tubes are attached on the outside of the most dangerous side columns to serve as wall purlin. These horizontal smaller tubes provide lateral braces to side columns in practice, but this effect is lacking of theoretical supporting. The paper carried out an analysis of the efficiency of the tube in providing the lateral support. Analysis revealed that, though the smaller tubes are eccentric to the centroid of the column, the lateral braces to the column could still be effective. The out of plane effective length could be decreased accordingly. According to this conclusion, the out of plane effective length could take the distance between the lateral braces when checking the stability of the column. In this way, the checking calculation of the side column could be more scientific and reasonable on the bases of less material application and proper strength and stability of the side column.