Quantum cascade laser (QCL) has been used in many fields for its characters of small size, small weight, high efficiency and direct modulation. The output laser beam of the QCL is nearly to be Guassian style. The evaluating method of the laser beam quality is analyzed in theory based on the Gaussian beam transmission law. And the output near field and far field of the single quantum cascade laser is measured in the experiment. The output divergence angle is calculated and the output beam quality is analyzed by using the M2 factor.