We have experimentally implemented a difference-frequency-generated mid-infrared (MIR) source based on synchronously pulsed pumping. The passive synchronization between polarization-maintaining Er- and Yb-doped mode-locked fiber lasers was realized by using all-optical modulation technique via master-slave injection. By combining spectral broadening of highly nonlinear fiber and wide-band tunable filter, we could obtain broadly tunable MIR picosecond laser from 2940 nm to 3260 nm. The average power was 580 mW to 926 mW,and the maximum pump conversion efficiency was 41%. In the experiment, the injection of low-power synchronous induced pulses could substantially reduce the pumping threshold for the MIR parametric generation, which would relax the requirement of strong pump field to realize efficient MIR generation.