A new quantum key distribution scheme is proposed to realize both phase coding and time coding based on two unbalanced Mach-Zehnder interferometers. The pulses which are discarded in the phase coding scheme can be coded in time coding, so the useful bit rate in the present scheme can be doubled. At the same time, the phase coding keys and the time coding keys are obtained. We can use one group of keys to communicate and the other one to guard against eavesdropping, or combine both of them to form new keys. This scheme has favorable application prospect.