According to the present situation of the low mechanization of field experiments and the lack of plot equipment in China, in order to adapt to the operating environment in hilly and mountainous areas and meet the harvest requirements of super hybrid rice, the 4LZ-21Z type combine harvester was developed. The integrated design of combine harvester and doublespeed doubleaction threshing and separating unit was introduced. The grain plot combine had the functions of holding and dividing grain stems, cutting grain stems, threshing and cleaning of grain and bagging seeds. Low/high speed threshing drum speed, rotary concave screen speed and threshing gap were used as test factors, and grain loss rate, broken rate and impurity rate were used as performance indexes. The experiment was designed by threefactor fivelevel orthogonal rotation combination. The DesignExpert software was used to optimize multiobjective variables, the mathematical model of each test factor and performance index were established, and the multiobjective parameters were optimized. According to the results of parameter optimization, field test of prototype was carried out. Field experiments showed that the performance of the prototype was stable, and the grain loss rate, broken rate and impurity rate were 1.34%, 0.20% and 0.40%, respectively. With each performance index superior to the test standard, this device solved the contradiction between impurity removal, entrapment and grain breakage loss during the harvest of “Yongyou 15” super hybrid rice effectively. This study was of benefit to improving the efficiency and precision of plant field experiments.