The distribution of transverse modes of the laser, especially the ratio of fundamental mode is proposed to evaluate laser beam quality, and the measuring method is studied. The beam spot images captured by a CCD detector are input into a nonlinear network which bases on the Hopfield network principle. The dynamics steady state is obtained by computing the energy function of the network and performing training arrangement. The proportion of all ranks of transverse mode in the steady state is the measurement result. During the experiment, a laser diode-pumped solid-state laser was used to emit laser beam, and the captured beam spot images were input into the nonlinear network after image processing, and the proportion of fundamental mode was 69%. The results were used to compose a new spot digital image, and the relative error compared the original image was 3.53%.