The invention relates to a non-whole-course double-circuit parallel power transmission line non-synchronous data fault distance detection principle. The non-whole-course double-circuit parallel powertransmission line non-synchronous data fault distance detection principle is characterized in that utilizing an electric transformer at a protection mounting position to acquire the electrical data ofeach end of a system, respectively decoupling a double-circuit coupling line section and a single-circuit line section, calculating the positive sequence voltage of each end, and the whole power andthe fault component of the current of the line; according to the magnitude, of the positive sequence voltage fault component of the connection point, calculated at two ends of the same line, determining the fault branch circuit; and on the fault branch circuit, according to the double end non-synchronous fault distance detection principle to position the fault so as to eliminate invalid false rootand determine the fault position.