The invention discloses a method and system for backstepping the sound power of a single-phase wire through 100 Hz pure sound of a transmission line. The method includes the steps that pure sound monitoring points are arranged on the lower section of the transmission line, the sound pressure level of the pure sound is measured, and the precipitation generated when the sound pressure level of the pure sound is measured is obtained; the 1/3 octave is adopted for extracting the component of the 100 Hz pure sound, and sound pressure corresponding to the sound pressure level of the pure sound underthe heavy rain precipitation condition is counted; the distance coefficient is calculated according to the tower type and the above ground height of the transmission line; a simultaneous equation ofthe sound pressure and the distance coefficient is established, and the pure sound power yield is obtained by solving the equation.