Mode division multiplexing technology can achieve the co-fiber-transmission technology of quantum signals and classical optical signals. A quantum key generation rate model with simultaneous nonlinear effects and mode coupling is constructed for a quantum mode division multiplexing simultaneous-transmission system. The relationship between the quantum key generation rate and the communication distance is studied by simulation calculations. Simulation results show that the maximum safe communication distance of the system is too short, and there is a big gap compared with the performance of traditional quantum key distribution systems. Accordingly, a quantum mode division multiplexing simultaneous-transmission scheme based on double decoy states is proposed to increase the maximum safe communication distance of multiplexing simultaneous transmissions. Moreover, to make full use of fiber channel resources, multiplex quantum signal multiplexing transmission is performed on an idle mode channel, which improved the communication performance of the quantum mode division multiplexing simultaneous-transmission system and narrowed the gap with the traditional quantum key distribution systems.