According to the features of large crossing of stain-tangent-strain style at Heiyanzi of the Yangzi River,the catenary control method is applied,by which sub-conductor sags are observed and the removed-length is calculated by ground marking.It is shown from the repetition measurement of assembled-conductor sags that the sag measurement and the secant calculation method are efficient for the conductor sag and sub-conductor errors meeting the demands of design requirements.