The general value ranges are not given in the standard for the selection of the beam height of the main girder of the hydraulic plane steel gate, and the theoretical solution formula that does not consider the main beam and other components work together is taken as a reference in the actual projects. Under the condition that the stability requirements are met and the components such as the panel and the partition plate work together with the main beam, the finite element analysis theory was used to establish the finite element model of the plane steel gate space, the variation law of the strength and stiffness of the main beam was calculated and analyzed, and the optimum beam height of the main beam of the plane steel gate was obtained. The results showed that the stress of the main beam is more suitable when considering the cooperative work of each component. The optimal beam height exists between the minimum beam height and the economic beam height, and the ratio of the optimal and minimum beam height is about 1.038. Compared with the excellent beam height from the theoretical formula, the stress of the main beam flange and the web can be reduced, and the working performance of the main beam can be optimized at this beam height. The results in this paper can provide reference for the design of plane steel gate height.