In this paper, axial compression tests were firstly performed on Q345 grade H-shaped steel columns. Based on systematic theoretical deduction, a computational method was then presented to evaluate the ultimate strength loss of the pitted steel column with H-section under axial compression. The detailed formulas were calibrated by numerical simulations, and the random pitting size and locations were both considered. The influence of a single pit on the column strength loss was firstly investigated, and the contribution function of a single pit was established. Then the interaction effects between pits were investigated by considering the relative size, distance, and angle of the pits, reflected by the interaction coefficient. A family of finite element models with consideration of the randomness of corrosion pits were built to validate the feasibility of the proposed computational method.