We have experimentally investigated mid-infrared (MIR) difference-frequency generation (DFG) based on passive all-optical synchronization. Synchronized pulses were obtained by master-slave injection locking between all-polarization-maintaining Yb- and Er-doped fiber lasers, which were amplified by cascaded fiber amplifiers to realize MIR picosecond pulses at 3071 nm within a PPLN crystal. The maximum pump conversion efficiency is 68.3%, and the peak average power reaches 1.36 W. The results show that the synchronous pulse induced DFG can substantially reduce the pump threshold. Additionally, benefiting from the all-polarization-maintaining fiber architecture for the pulse generation, synchronization and amplification, the MIR ultrafast light source exhibits excellent long-term stability with a relative fluctuation of average power as low as 0.2%.