Nd3+ :GSAG, a laser crystal of 942 nm with good performance, was grown by Czochralski method successfully, and its transmission spectra at room temperature were studied. Sixty-eight crystal field splitting levels of Nd3+ with the maximum up to 29967 cm-1 were identified, to which the Hamiltonian parameters of free-ion and crystal field were fitted, and the root-mean-square deviation of energy level fitting is 16.7 cm-1, which indicated that the experimental and calculated energy levels are well consistent. The obtained Hamiltonian parameters can be used to calculate the wavefunctions of Nd3+ in GSAG, predict its energy level splitting, study the luminescent properties and so on.