The room-temperature continuous-wave tunable narrow band coherent light source in the mid-infrared region is valuable for trace gas sensing. Nonlinear optical difference-frequency generation (DFG) scheme has been used as a useful way to meet the requirement, and complementarity for traditional laser technology. The temperature and signal light wavelength tuning characteristics of quasi-phasematching (QPM) based DFG system were theoretically studied, the analytical expressions of temperature and signal light wavelength tuning bandwidths of periodically poled LiNbO3(PPLN) were deduced based on the general theory of QPM. A widely tunable room-temperature mid-infrared laser source system based on QPM in PPLN crystal has been developed, and the mid-infrared coherent radiation is produced from 3.2 to 3.7 μm with a maximum output power of about 1 μW. The temperature and signal wavelength tuning characteristics of the system were experimentally studied, and the experimental results were compared and analyzed with the theory.