Abstract: The error of results in simultaneous spectrophotometric determination is discussed. The condition number of the coefficient matrix has a great effect on the calculated results. For an ill-conditionned matrix, you can not ensure reliable results by use of a non-negative contraint. In these conditions, addition of stricter constraint is necessary.Anew constrained optimization, the improved alternating variate method is described. By this method it is possible to add arbitrary constraints for any variate, and it is found satisfactory to the systems in which the condition number of coefficent matrix is larger.The method is applied successfully to the simultaneous spectrophotometric determination of light rare earth elements La, Pr, Nd, Sm. Using the data of general accuracy (experimental erroe≤3%), and satisfactory results can be obtained.Applying two systems in measurement(using chloro-phosphonazo and carboxynitrazo as chromogenic reagents) is suggested to improve the condition number of the coefficient matrix.