The invention provides a hard surface rebuilding method and system for stainless steel branch pipe welding seam repair. The hard surface rebuilding method comprises the following steps of acquiring defect information, original data and field operation environment data of a branch pipe welding seam to be repaired; according to the acquired defect information, original data and field operation environment data of the branch pipe welding seam, formulating a hard surface rebuilding overall scheme; performing treatment and surfacing on the branch pipe welding seam to be repaired according to the formulated hard surface rebuilding overall scheme; and performing inspection, testing and evaluation on the surfacing welding seam. In the method, by directly performing surfacing on corrosion-resistantmetal on the surface of an to-be-repaired area of the branch pipe welding seam, a new pressure boundary is formed by a surfacing layer, the structure of the branch pipe welding seam is strengthened,the state and distribution of residual stress in the branch pipe welding seam are improved at the same time, and the area nearby a defect area subjected to surfacing is promoted to be in a pressure stress state, growth and expansion of cracks are inhibited, and the repaired branch pipe welding seam is guaranteed to meet the full-cycle safe operation requirement.