Abstract: On the base of an orthogonal experiment of sixteen groups of frame column double beams underpinning joints, the three factor influence on the underpinning joint bearing capacity was studyed, the three factors included the ratio of span to height of a walking beam, the ratio of an underpinning beam stirrup steel bar, and the ratio of an underpinning beam longitudinal steel bar. Based on the experimental results, the underpinning joint mechanism and failuremodes were determined, and the conclusion was drawn that the ratio of the walking beam span to height was the key factor to the jiont bearing capacity; the failure course was divided into two stages before and after interface punching slip by theoretical analysis. For the stage before interface punching slip, a space “tension-bar-arch” mechanical model was presented for the frame column underpinning jiont, then mechanical analysis was put into the model, and the thereotical caculation formula for frame column underpinning joint was obtained. At last this thereotical results were compared to the experimental results, and the thereotical results were good agreement with experimental resuls.