HVDC plays the more and more important role in China's power system because of its large transmission capacity and long transmission distance. Once the fault occurs in DC transmission line, it is very important to identify the fault quickly and accurately. In this paper, the traveling wave protection scheme of DC transmission line based on wavelet transform is studied systematically. It is found that the sampling range of the same traveling wave can cause the fluctuation range of the modulus maximum of the wavelet transform to be more than 7 times under the sampling rate of 10 kHz, which means that the discrete sampling protection setting has serious numerical stability. This paper proposes a multiple sampling method. Taking the sampling weight as the characterization index, the possible range of wavelet transform fault index numerical wave is obtained, and the probability distribution of the wave value is analyzed. On this basis, the influence of the transition resistance value on the fluctuation range of the fault index is obtained. The research methods and conclusions of this paper have important reference significance for the traveling wave protection setting in practical engineering. This work is supported by Science and Technology Project of State Grid Corporation of China (No. XT71-17-028).