An optical parametric oscillator based on B a G a 4 S e 7 crystal is constructed for use as a 3-8 µm mid-infrared source. The output characteristics of the light source, including wavelength tuning, beam quality, and energy stability, are studied, which are usually concerned differently in spectral analysis, imaging, and laser processing. When the light source operates far from the threshold, the mid-infrared output is poor in beam quality and good in energy stability, and the contradiction should be addressed in practice. The results provide guidance on the selection of operating parameters and performance optimization for the application of the B a G a 4 S e 7 based mid-infrared source in various settings.