The bit error rates (BERs) of both the free space quantum communication system and the classical optical communication system are analyzed. A free space quantum-classical signal coexistence transmission system is designed based on the wavelength division multiplexing technology, whose feasibility is investigated by the software simulation. The 15 bits quantum information and 128 bits classical information are totally transmitted within 256 ns and the BER of classical information is 4.07×10 -15. The results show that the designed system can effectively realize the coexistence transmission of free space quantum and classical signals in the same channel and simultaneously expand the channel capacity of the free space quantum key distribution system.