The invention discloses a mixed modulus time domain fault positioning method for an identical-tower double-loop direct current line. The method comprises the following steps that 1, a phase-mode transformation matrix is constructed through an impedance matrix and an admittance matrix, and a voltage decoupling matrix and a current decoupling matrix are built; 2, according to the obtained voltage decoupling matrix and the current decoupling matrix, the voltage and the current obtained at the two ends of the line are subjected to decoupling conversion, each modulus voltage and modulus current of the identical-tower double-loop direct current line are worked out, and the line modulus voltage component and the line modulus current component are extracted; 3, the way-side voltage distribution at the two ends of the line is calculated; 4, the fault positioning criterion based on the mixed modulus is constructed. The mixed modulus time domain fault positioning method has the advantages that the fault positioning precision is high, the reliability is high, the calculation quantity is small, the realization is easy, and the like.