While calculating the aggregate proportion in the ratio of mixture design by normal equation method, the value of solution may be negative, or the result gradation of the compound may go beyond the standard range. By iterative algorithm, the improved method uses the gradation data calculated by normal equation method as initial target value, canceling the relevant aggregate if the result of proportion is negative, and setting the gradation to the boundary value if the sieving data goes beyond limits. According to the adjusted object value, using the normal equation method again, the new aggregate proportion can be solved. Then the new gradation of the compound will come into being as well as the next target value. Finally, the accurate result which is up to the mustard can be found by the iterative algorithm. Although the principle of the improved method is simple, it has good applicability in the engineering and other excellent characteristic, such as good astringency, stability, convenience, and the same accuracy compared with the quadratic programming method.