The invention relates to an optimal design method of a smoke box structure of a large electric precipitator. The method includes: optimally designing distribution of reinforcing ribs of the smoke box structure: a design model only comprises a wall plate and a supporting rod, a self-adapting growth method based on the nature branch net morphosis mechanism is adopted, and distribution of the reinforcing ribs of a smoke box is designed with minimum structure strain energy as the design target and under the condition of reinforcing rib volume limiting; and optimally designing dimensions of structure parameters of the smoke box in details with minimum structure mass as the design target and under the condition of meeting structural strength and rigidity requirements, and therefore an optimum smoke box structure with high rigidity and strength and low mass is obtained.