According to the relationship between fiber strain and Bragg wavelength, the calculation methods of sags are obtained under the conditions of isoheight and non-isoheight suspension center of overhead transmission line. A real-time monitoring scheme of sags on power transmission lines based on fiber Bragg grating (FBG) strain sensors is proposed. As a result, the real-time sag values can be determined by the strain of the monitoring lines. It is shown from the simulation that the scheme is applicable to the real-time monitoring of sag variation more than 10m, and the accuracy of measurement reaches to 16 mm, which has excellent feasibility in the project application.