Based on the fact that the current bridge reinforcement design specifications do not see the reinforcement of I-section eccentric compression members by increasing the cross-section method, the main calculation process of I-section eccentric compression members based on the Specifications for Strengthening Design of Highway Bridges (JTG/T J22-2008) was supplemented and improved in this paper, the calculation formula of key parameters were obtained. The coefficients A, B, C and D of the univariate cubic equation for solving the height x of the compression zone under different eccentric conditions were determined. In order to get the nominal strain ε^′_s2 which corresponding to the new longitudinal ordinary steel bar A_s2 in the first stage, the coefficients A, B, C and D of the univariate cubic equation for solving the first stage compression zone height x₁ of the original I-section member under the large eccentricity were determined. This paper provides a reference for the bearing capacity calculation of the same type of members strengthened by the enlarged section method.