The invention discloses a method for calculating reliability of a flexible direct-current (DC) transmission system of a large wind farm. The method comprises the steps: firstly, dividing the system into a plurality of subsystems according to structural characteristics and operation modes of the DC transmission system, and determining a reliability structure block diagram of the DC transmission system; establishing a reliability evaluation module of each subsystem, respectively enumerating the running state of each subsystem, finally, calculating the reliability index of each subsystem by adopting a frequency duration method, and calculating the reliability index of the system according to the reliability structure block diagram of the system. The method has the remarkable effects that the frequency duration method and a state enumeration method are combined organically, so that the quantity of calculation is small, the influence caused by the intermittent output and fan failure of the wind farm during evaluation can be taken into account, and the universality is relatively good.