Although classical-quantum channel multiplexing technology attempts to improve the channel use of quantum secure communication, noise crosstalk severely limits the quantum channel transmission efficiency. A combined optimisation method is used to optimise the channel allocation strategy based on channel Raman noise interference analysis, and an optimal band selection algorithm suited for the quantum key distribution system is proposed. The allocation of classical and quantum channels is adjusted and optimised using the proposed combinatorial optimisation algorithm based on the correlation between the noise and channel relative position to obtain a multi-band multiplexing scheme that minimises noise. Compared with the traditional dual band multiplexing scheme, the optimal multi band multiplexing scheme can effectively reduce the interference of classical light to quantum light between different frequency bands, so as to improve the key rate of channel transmission.