Quantum cascade laser (QCL) emitting in the mid-wave infrared atmospheric windows (3 μm to 5 μm) will be of immediate use to several civilian applications, including airborne self-defense protection system and trace gas sensing and free space optical communications. At present, the mid-infrared laser sources mainly include solid-state optical parametric oscillation lasers, fiber lasers, and QCL. In these lasers, quantum cascade laser is the only one that can realize the conversion from electricity to light. Since its invention in 1994, with the deepening of scientific research, quantum cascade laser performance has been continuously improved, and the output laser power and beam quality of single transistor has been continuously improved. In this work, the output beam quality of QCL is analyzed. the evaluating method of the laser beam quality is analized in theory based on the Gaussian beam transmittion law. The output nearfiled and farfield of the single quantum cascade laser is measured in the experiment. The output divergence angle is calculated and the output beam quality is analized by using the M2 factor.