At present, there are several problems in determining the height of main beam in hydraulic steel gate, including a large number of empirical formulas, imperfect theories, and ignorance of influences of section form and support position. According to design code of steel gate, with requirements of strength, stiffness, stability and geometric structure as constraint conditions, we take minimum steel amount of main beams and vertical baffles as objective function, and establish optimization models for beam height of simple support and double cantilever’s girder under conditions of uniaxial and biaxial symmetrical sections. Theoretical formulas of optimal girder height are derived by semi-analytical methods. Through comparing with built projects, we find that girder heights calculated by the theoretical formulas in this paper are close to those in practical projects, which still can be optimized. Furthermore, we can decrease beam height through reducing height-thickness ratios of partition and web of main beam. The theoretical formulas can be applied to engineering practice, and also provide a theoretical basis for improving specification.