The invention discloses a method for calculating reliability of a high-voltage direct-current power transmission system based on a universal generation function, comprising the following steps: assuming that elements in the high-voltage direct-current power transmission system are functionally independent of one another, and subjecting a structure diagram of the high-voltage direct-current power transmission system to equivalent transformation; subjecting the high-voltage direct-current power transmission system subjected to equivalent transformation to serial-parallel structural decomposition so as to obtain subsystems of the system; acquiring element reliability parameters in the high-voltage direct-current power transmission system, and calculating an element state-probability distribution function; subjecting the element state-probability distribution function to Z transformation to calculate an element U function; calculating the U function of each subsystem according to a functional function; calculating a state-probability distribution function of the high-voltage direct-current power transmission system according to an equivalent structure diagram of the high-voltage direct-current power transmission system. The invention provides state-probability distribution function calculation for HVDC (high-voltage direct-current) power transmission system in time domain, thus pre-judging real-time reliability and laying basis for the formulation of corresponding maintenance plans and optimization and decision.