We carried out horizontal low cyclic loading tests of four all-welded double beam-column inner joints to study the seismic performance of the double beam-column inner joints of the steel intimated ancient building. We studied the mechanical performance of the double beam-column inner joints according to the loading process and failure pattern of the specimens under the action of lateral forces, and divided the double beam-columns into two kinds,which were the box section beam and I-shaped section beam,according to different section forms of beam.Based on unique structural features of the intimated ancient building,we divided the joint core area of each specimen into three regions,which were upper core zone, middle core zone and lower core zone. By measuring the shear strain within the scope of every area,we analyzed the mechanism performance and failure modes of the joints under lateral forces, and established the diagonal strut mechanism. The experimental results and theoretical analysis show that the failure mode of the double beam-column joints of the intimated ancient building is mainly for the shear failure along the diagonal of the lower core area. So we presented a modified formula of shear bearing capacity of double beam-column inner joints considering the axial compression ratio and the cross section of the beams.