A quantum private communication system based on the wavelength-mode division co-multiplexing is put forward and designed. The basic principle of wavelength-mode division co-multiplexing is introduced. The transmission experiment is performed for 10 times with the system model built by Optisystem. The time domain waveform and spectra of signals are obtained and analyzed. Stabilities of wavelength and linear polarization mode of signals in the transmission are tested. The quantum key distribution based on the scheme of double unbalanced Mach-Zehnder interferometers is realized in the system. The error bit rate, eye diagram and quality factor of classical optical data are obtained. The results show that this quantum private communication system converges the advantages of two multiplexing forms. The channel capacity is broadened, and the isolation and the orthogonality between signals are enhanced. In addition, the nonlinear effects and the crosstalk between signals are reduced efficiently, and the quantum signal is well maintained in the coexisting transmission process of quantum-classical signals. The proposed quantum private communication system is confirmed to be effective and applicable.