A widely tunable mid-infrared difference frequency generation (DFG) laser source is reported which uses ytterbium doped fiber lasers (YDFL) and an erbium doped fiber laser (EDFL) as the fundamental lights respectively. By using an electronically controlled optical switch to choose one of different tuning bands respectively supplied by four tunable YDFLs and to quickly switch among them, the whole tuning range of the pump light is broadened to be between 1040nm and 1110nm. Two electronically controlled polarization controllers (ECPC) respectively operating in 1060nm and 1550nm waveband are also used to precisely adjust polarizations of the pump and signal lights, so as to improve the conversion efficiency of the DFG system. The PID algorithm and feedback technique are used to control the temperature and driving current of the pump LDs, the controlled conditions are displayed with an LCD. The measurement results show that the mid-infrared laser system may be continuously tuned within the range of 3.04~3.72 μm.