The invention discloses a method for improving the power transmission capacity of the tie line of an alternating-current system under a multi-loop direct-current fault. The method includes the following steps that: a three-area alternating current-direct current interconnection system is established; the influence of a generator spinning reserve on power transmission capacity is obtained through an alternating-current tie line between a system A and a system B; based on the two-unit interconnection equivalent system of the system A and system B, the influence of the quantity of direct-currentpower disturbance between the system A and a system C on the power transmission capacity is obtained; the influence of different generator rotational inertias on the power transmission capacity of a weak section is obtained through the three-area alternating current-direct current interconnection system; the influence of the generator spinning reserve, the direct-current power disturbance quantityand generator rotational inertias on the power transmission capacity of the weak section is analyzed, so that the power transmission capacity of the tie line of the three-area alternating current-direct current interconnection system is improved. According to the method of the invention, on the basis of the simulation analysis of an actual power grid, the accuracy and correctness of referred influence factors are verified.