A room-temperature widely tunable mid-infrared laser spectroscopy system based on difference frequency generation(DFG) developed in our laboratory recently is reported. The system employs two nearinfrared diode lasers ranging in 1490～1580 nm and 1058.1～1060.8 nm as signal and pump sources, respectively.PPLN crystal is used as nonlinear mixer unit, and the continuous coherent light output tuning from 3.2 μm to 3.7 μm is achieved combining quasi-phase matching technique. The output bandwidth characteristics of the system is studied experimentally and compared with the theoretical calculation.