The invention relates to a string weight tension sag algorithm. The algorithm is conducted on the basis of actual parameters. The algorithm includes the following steps of calculating the horizontal stress allowed under various working conditions, calculating the critical span Lcr of any two types of working conditions, calculating the span range corresponding to each working condition, calculating the representative span L, obtaining the working condition and horizontal stress corresponding to the representative span, calculating the horizontal stress under other working conditions, calculating the three-section catenary span and the elevation under each working condition, and calculating the sag curve of the three-section catenary under each working condition. Based on a catenary model,considering the length and weight of a strain insulator-string, the string weight tension sag algorithm is put forward, the tension sag under various working conditions (considering wind speed influences) can be precisely calculated, and the method is used for calculating the tension sag and string weight sag of a strain insulator-string gear with wires in a power transmission line.