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China has superior natural conditions and is suitable for the growth of fruit trees. It is a big fruit producing country. The cultivation area and yield of fruit trees rank first in the world all year round. Fertilization of fruit trees is a key link in orchard management. Ditching fertilizer machine is an important equipment to realize mechanized fertilization in orchard. In order to improve the dynamic performance of orchard ditching fertilizer machine, avoid resonance, and ensure ditching consistency and fertilization stability, the multi-objective optimization design of its rack was carried out. Firstly, the parametric model and finite element model for the rack of the orchard ditching fertilizer machine were established. Secondly, through modal analysis, the natural frequency and vibration mode of the rack were obtained, and its influence on the dynamic performance of the whole machine was studied, and the first-order modal frequency was set as the objective function. Through the sensitivity analysis of the rack, the sensitivity of each member thickness to the first-order modal frequency was obtained. The thickness of the sensitive member was set as the design variable, and its thickness variation range was set as the constraint condition. According to the requirements of modern agricultural machinery structure lightweight design, the quality of the rack was also taken as the objective function. On the basis of objective function, design variables and constraints, the mathematical model of multi-objective optimization of rack was constructed. Then, based on Hammersley sampling method, totally 42 groups of test samples were selected according to the constraint conditions to design the experiment, and the corresponding target values were calculated. According to the experimental design results, the moving least square method was determined and the corresponding response surface was fitted. Among them, the complex correlation coefficient R2 of the first-order modal frequency response surface model was 0.9974, and the complex correlation coefficient R2 of the mass response surface model was 0.9999, which were all higher than the accuracy required by the fitting model of 0.9, which met the design requirements. Finally, based on response surface methodology and multi-objective genetic algorithm, the structure optimization design for the rack of the orchard ditching fertilizer machine was carried out. The optimization results showed that the first-order modal frequency of the optimized rack of the orchard ditching fertilizer machine was increased by 8.25% from the original value of 35.39Hz to 38.31Hz, and was far away from the input frequency of the tractor;the optimized mass was 389kg, which can meet the requirements of minimum mass when the first-order modal frequency was raised. After optimization, the ditching consistency and fertilization stability of orchard ditching and fertilizing machine were increased by 3.72% and 3.57% respectively compared with those before optimization, and the improvement effect was obvious, which met the requirements of orchard ditching and fertilization production.