Abstract: The laser intra-cavity absorption method is of a newly developed technique used for the determination of weakly absorbed substance, it has the merits of high sensitivity and simple operation. This paper has, studied the effects of various laser parameters(including resonant cavity length, pump energy, the transmittance of output mirror, pulse repetition frequency, etc.) on sensitivity. Apreliminary theoretical exploration is presented. An important reason why the formula proposed by Horlick does not coincide with the result of this experiment is due to the fact that two reflective mirrors of the resonant cavity can not be strictly parallel. The corresponding formula is deduced.Experimental results showedn that Beer's law is obeyed in the range of Neodymium ion concentration of 2 ×10^-3-1×10^-2 M, the sensitivity increases by a factor of 27, as compared with that of the conventional absorption spectrometry, some rare-earth elements like La, Ce, Pr, Sm, Yetc. do not interfere. This method can be used for the determination of Neodymium in light rare-earth mixtures.