The mixed damage of Sphacelotheca reiliana （A）, Curvularia lunata （B） and Ostrinia furnacalis （C） in three maize varieties was studied in Shenyang, Liaoning Province, by applying complete blocking and general quadratic rotation designs. Results showed that the ears weight has curvilinear correlation with each ear midge and there was highly significant damage of A B C and ABC to the maize yield loss. The total yield loss of mixed damage did not have relation to varieties. At the end a simulated model of mixed damage and economic threshold was developed, which is adaptable to field reality in the plot and block field trials.