The environment of ultrahigh-voltage long-distance hybrid cascading overhead dc transmission lines is complex, and the line fault probability is high. The transmission time of the conventional pilot protection via optical fiber communication is long, causing delays in the implementation of protective actions and seriously affecting the protection scheme's responsiveness. This article, an accelerated backup protection suitable for long-distance dc transmission lines is proposed. Using the Chebyshev digital filter to extract the high-frequency components of the fault voltage, the protection scheme uses the ratio of the components on both sides of the electrical boundary of the dc transmission line to formulate a directional element. Based on the directional elements on both sides of the line, a blocking-type pilot directional protection is established. Experiments show that the backup protection scheme can reduce the protection time compared with the traditional pilot protection, and also has the ability of accurate fault pole selection. The protection principle and method have strong robustness to transition resistance and noise.