Abstract: In the present paper, it has researched the effect of co-color action on the sperctrophotometric analysis of the rare earth. It has been discovered that when lighter and heavier rare earth are coexisted the absorbance of complex of the heavier rare earth with chlorophosphonazo-Ⅲ obviously increases under a certain experimental condition and that the absorbances appear a regular change with increasing of content of the lighter rare earth. We have probed into using the method of regression analysis on the microcomputer of TRS-80 type for treating experimental results. Namely, these two regression equations are built up for λ_max of a and β peak, respectively. Therefore, after these two absorbance values were measured on these two λ_max for single sample and inputting into microcomputer it is able in a measured result to predict the content of the lighter and heavier rare earth and to cancel partial effect of the co-color action. The self-edited computer program with BASIChas passed through active state examination. With microcomputer to carry out regression treatment the result has shown that the use of chlorophosphonazo-β for determining Yin the presence of La may decrease measured error from +26±15% to +3.6 ±3.0%. Above method is simple, fast and convenient. In principle, the accuracy consists with requirement of spectrophotometric analysis.