Abstract: With the development of the high-head hydropower projects, the application of the deep-hole gates is becoming more and more popular. In this situation, the design of the main beams of the deep-hole gates has already belonged to the range of the deep-beams. If the pure bending theory of slender beams in traditional design is still used, the calculation results will be erroneous. Therefore, it is necessary to give a concise and accurate method for calculating the normal stress of the main beams of the deep-hole gates. In this paper, based on the superposition idea and material mechanics method, considering warpage and longitudinal extrusion effects, a reasonable mechanical calculation model is developed, and the analytical formulae for calculating the positive stress of the I-section deep beam under lateral uniform loads are derived. And the analyses of the relative errors and applicable ranges between the calculated results and the existing calculation methods and numerical simulation under different span-height ratios and flange-web area ratios are carried out. Then the accuracy evaluation of different calculation methods is given with case histories in the practical engineering. The analysis and calculation results show that compared with the existing analytical methods, the calculation results in this paper have higher accuracy and wider application, and can provide a technical reference for the design of the deep-beam structures.