The conversion efficiency properties of a widely tunable continuous wave Mid-IR fiber-type laser source based on difference frequency generation (DFG) process in a periodically poled MgO-doped LiNbO3 (PPMgLN) crystal, which uses a ytterbium-doped fiber laser (YDFL) as the pump and an erbium-doped fiber laser (EDFL) cascaded by an erbium-doped fiber amplifier (EDFA) as the signal respectively, are investigated in this paper. Our results show that, the polarization states of pump and signal beams influence the conversion efficiency of the DFG process. By using polarization controllers (PCs), the polarization orientations of pump and signal beams may be respectively adjusted to be parallel to the principle optical axis of the crystal for obtaining high conversion efficiency. The beam quality of the pump and the signal not only influence on the conversion efficiency of the DFG process, but also determine the tolerance of the crystal position along the beams. For a focusing system formed by a fiber grin lens and a convex-plane lens with the focal length of 100mm, the conversion efficiency and the tolerance for the DFG source are 0.717 mW-2 and 44 mm, respectively. Moreover, the conversion efficiency of the DFG process changes very small when the idle light is tuned in the wavelength range between 3126.36 nm and 3529.6 nm.