We propose a fully symmetrical QKD system that enables quantum states to be prepared and measured simulta-neously without compromising system performance.Over a 25.6 km fiber channel,we demonstrate point-to-point QKD operations with asymmetric Mach-Zehnder interferometer modules.Two interference visibilities of above 99％ indicate that the proposed system has excellent stability.Consequently,the scheme not only improves the feasibility of distributing secret keys,but also enables QKD closer to more practical applications.