The invention relates to a power transmission line early fault detection method based on a Kalman filter. The method comprises the following steps: carrying out Kalman filter slow dynamic estimation on a monitoring current signal of a power transmission line to obtain an estimated current signal; comparing the monitoring current signal of the power transmission line with the estimated current signal, and serving the difference value between the monitoring current and the estimated current as a first standard signal; comparing the first standard signal and a fault state current threshold value of the power transmission line and making a preliminary diagnosis according to the comparison result; carrying out Kalman filter fast dynamic estimation on a monitoring voltage signal of the power transmission line to obtain an estimated voltage signal; carrying out square wave fitting on the estimated voltage signal, and serving the difference value between the absolute value and the mean value of a square wave voltage signal obtained after fitting as a second standard signal; and comparing the second standard signal with a mild early failure state voltage threshold value of the power transmission line, and confirming or denying the preliminary diagnosis according to the comparison result.