The multiplexed classical and quantum transmission was one of the key application technologies of quantum secret communications, which can solve the problem that quantum information and classical information need different optical fibers to transmit, and significantly reduce the application cost. Aiming at the scheme of multiplex continuous variable quantum key with classical information based on WDM technology, characteristics of Raman scattering noise in the system are analyzed quantitatively. Influence of Raman scattering noise on the system security key rate is studied by simulation under two different classical information transmission modes of forward and backward. Results show that the secure key rate in the forward classical transmission mode system is larger than that in the backward classical transmission mode system significantly. When the channel input power is fixed, the influence of Raman scattering noise on the security key rate is small in short range communication, and the influence is not negligible with increasing of the distance. When the communication distance is fixed, the additional noise in a certain numerical range has less influence on the system security key rate, and the value is 0.07N0 when the communication distance is 50 km.