By tracking phase drift and distributing key at suitable times, the quantum key distribution based on double M-Z interferometer was performed over a fiber length of 80 km with a quantum bit-error rate of 6%, and it had been working stably for several days. In addition,we demonstrate that the quantum bit-error rate is less than 3.5^ within a fiber length of 50 km. Through the local-area network,the key exchange and document transmission were achieved. The scheme applicable to other QKD systems can also improve the stability of interference,which has made progress in the practical application.