Existing transmission line fault type and characteristic distinguishing methods are all based on variation features of electrical quantities passively, on some operation conditions, mal-operations or miss-operations are caused frequently because of lacking initiative. By drawing from power line carrier communication technology of longitudinal differential protection, and through analysis on transmission characteristics of high-frequency signal under various types of transmission line faults, an active protection scheme of transmission line based on high-frequency signal is proposed. The proposed protection scheme can distinguish fault type and characteristic actively by emitting high-frequency signal to transmission line. Results of simulation show that different types of faults can be detected by the proposed active protection scheme accurately, and through delayed cycle-judgment, temporary faults can be distinguished from permanent ones reliably. This work is supported by National Natural Science Foundation of China (No. 51267019 and No. 51467019).