Spectral oharaoteristios and energy structure of crystal NdAl3 (BO3) 4 have been analyed by the crystal-field theory to calculate its crystal-field parameters. By using these parameters, the stark sub-level wave-functions of 4F3/2, 4I11/2 and 4I9/2 manifolds have been obtained. Based on Judd-Ofelt theory, the relative transition probabilities of 4F3/2 to 4I11/2 manifolds have been calculated approximately by means of experimental data of fluorescent braching ratio, and the palorization of these transitions is discussed. In this way, the experimental results of fluorescence and laser emission can be well explained.