The invention provides a transmission mode distribution method and system. The system is configured in a space division multiplexing (SDM) optical network. The SDM optical network comprises at least one switching node. The switching node is equipped with a mode converter. The method is characterized by, when the first switching node receives a network service, obtaining occupied bandwidth of the network service; according to the occupied bandwidth, controlling the mode converter at the first switching node to determine a target degenerate mode from a plurality of degenerate modes correspondingto a plurality of scalar modes of a current FMF link; and based on the target degenerate mode, distributing spectrum resources on the current FMF link for the network service. In the dynamic differentiated distribution process, the spectrum resources of each order of scalar mode on the FMF link are fully utilized, and normal service transmission is ensured, thereby ensuring no-crosstalk transmission of the network service simply and effectively, and improving network performance.