The invention relates to an under-light plate branch time gap time diversion optimization configuring method for an SDH system, mainly comprising: in mapping mode, determining position arranging mode from TU time gap signal; for a branch plate whose configuration is not finished, preferring a to-be-allocated AU whose occurrence frequency is maximum to enter time division; setting number of to-be-allocated AU data, and number of AUs marked in time division to the branch plate, and calculating number of lower buses to be occupied by TU signal units corresponding to these AUs; selecting idle time division module and selecting a time gap flow corresponding to the maximum of all matching numbers to enter and be stationed in the time division module; for all the other AU time gap flows, calculating matching numbers and arranging AU time gap flows meeting conditions to enter and be stationed in the time division module. And it makes improvement in time division resources utilization, savingtime division resources and cost, and raising service configuration success rate.