The typical noise in the multiplexed system based on dense wavelength division multiplexing (DWDM) was analyzed, and the scheme of multiplexed classical and quantum transmission system was proposed with a heralded single-photon source. Compared to weakly coherent pulses, the threshold-detection can be accurately controlled, and temporal-filtering can be achieved to reduce the noise jamming. On the other hand, the opening time of APD can be controled to avoid the noise and dark counts. The simulation results showed that the scheme of multiplexed classical and quantum transmission system can depress the impact of various noise, reaching the threshold of bit error rate at a farther distance compared to a weak coherent source. So the secure communication distance is increased in remote quantum key distribution communications.