In order to achieve low-cost, long-distance and large capacity data transmission scenarios, a fiber-optic bidirectional optical fiber transmission system without multiple in multiple out(MIMO) mode division multiplexing(MDM) adjustment bandwidth is designed. Using six modes to select the photon lantern(PL) as the mode multiplexer and demultiplexer respectively, LP21 and LP01 are selected two non degenerate modes as forward transmission channels, LP02 mode and LP01 mode as reverse transmission channels. At the same time, two degenerate modes of LP11 are selected as forward and reverse transmission channels through optical switch control, thus realizing 12.5 km bidirectional bandwidth adjustable transmission. The system provides a solution for low-cost, long-distance and large capacity data transmission scenarios.