Traveling wave fault location devices record fault data and non-fault interference clutter using low-threshold variation start. However, the main information of traveling wave ranging is from fault early instantaneous data. So the most basic condition of traveling wave ranging is to screen out fault early instantaneous data automatically. This paper proposes a method of sinusoidal fitting to automatically screen out fault early instantaneous data. The current traveling data recorded by traveling wave fault location device is divided into two sections, the sine fitting of two sections is calculated respectively. Then modal eigenvalues of two pieces are calculated according to the degree and amplitude of sine fitting. If the calculated modal eigenvalue is greater than the set threshold value, it is judged as a failure early instantaneous data that contains valid information. On the contrary, it is regarded as the non-fault early instantaneous data. A large number of calculations show that the proposed transmission line traveling wave data automatically screening method based on sine fitting is feasible, accurate and reliable.