It is played a very important role for the power system stability to detect the transmission line fault location accurately. Through using transient current signal, this paper presents a transmission line fault location detection method for the three-phase fault which based on the R-L transmission line model. In the method, filter out the fundamental frequency component of the current signal when the voltage dips firstly. Secondly, change the processed current signal into absolute value. Then, dispose the current signal through logarithmic transformation. Finally, after the logarithmic transformation, get the slope by mean of the data. According to the slope, transmission line fault location could be detected. Simulation results show that the method can easily and efficiently detect transmission line fault location. The method has the prospect of application and promotion.