The invention discloses a butt joint design method for implementing equal load carrying of an I-shaped center crack-containing butt joint, belonging to the field of welding technology and solving the problem that the load carrying capacity of the I-shaped center crack-containing butt joint is lower than that of base material. For the butt joint with the center crack being contained in weld seam and crack existing in base material, the method mainly comprises the steps of: calculating matching ratio of fracture toughness of the joint, obtaining the stress intensity factor of a base material area, determining the stress intensity factor of a weld seam area satisfying equal load carrying when cracks exist in both the base material and the weld seam area, obtain the K-factor formula of the I-shaped center crack-containing butt joint and obtaining geometrical parameter values of the desired weld seam. And for the butt joint with the center crack being contained in weld seam and no flaw in base material, the method mainly comprises the steps of: measuring tensile strength of base material metal and fracture toughness of weld deposit metal, obtaining the K-factor formula of the I-shaped center crack-containing butt joint, determining the conditions that geometrical parameters of the equal load carrying joint reach, and obtaining geometrical parameter values of the desired weld seam. The method is suitable for flat butt joint with double sides being welded.