The invention provides an online positioning method for cable partial discharge. Firstly, the propagation of a partial discharge pulse signal in a cable is simulated through an off-line experiment, the propagation law of a partial discharge pulse signal in the cable is obtained, that is, the voltage amplitude and pulse width of the partial discharge pulse signal is monotonously changed with the increase of a transmission distance, and a database of a corresponding relationship between the pulse width of the partial discharge signal and the transmission distance is established. Then a pulse width variation function relation formula considering frequency characteristics under an ideal condition is derived by time-domain and frequency-domain transformation. Finally, the database of the corresponding relationship between the pulse width of the partial discharge signal and the transmission distance and the pulse width variation function relation formula are combined, the online positioningmethod for cable partial discharge based on the cable frequency characteristics and the signal pulse width is proposed, and an experiment proves that the method has high accuracy and practicability.