We investigate the beam quality features of an Airy beam during propagation in free space. The beam propagation factor of an Airy beam is derived based on the moments method. The results show that the beam propagation factor of an Airy beam can achieve the minimum M² = 1.17 when the modulation parameter a = 0.756 by compromising the effects of lateral shift, diffraction, and the departure of the main lobe of the beam with respect to the origin. Numerical calculations provide an intuitive picture to describe the propagation of an Airy beam in more detail.