The invention discloses a forward-back substitution method for reliability assessment of complex radial power distribution networks, and belongs to the technical field of reliability assessment methods of power distribution networks. A computer is adopted, power distribution network data are input through programs at first, radial power distribution network branch arrays capable of being applied to forward-back substitution calculation are determined, then node reliability indexes are determined by utilizing the forward-back substitution method for calculating reliability indexes, and finally the reliability indexes of a system are calculated. The forward-back substitution method can be widely applied to accurate and rapid assessment of the reliability of large-scale complex power distribution networks with multiple standby power supplies and multiple branch lines, influences of disconnecting switches, branch line protectors, standby transformers, standby power supplies and other configured devices on the reliability of the power distribution networks can be calculated conveniently, the time for reliability assessment of the complex power distribution networks can be shortened effectively, and scientific bases are provided for plans and operation decisions of the power distribution networks. The forward-back substitution method can be widely applied to the reliability assessment of the radial power distribution networks.