Weld cracking is a predominant damage mode in tubular connections. Such welds tend to crack prematurely before reaching their ultimate load-bearing capacity, and the damage mode of cracked joints differs significantly from that of seamless ones. This study focuses on the deformation behavior of K-joints with weld cracking at the root of the tension branch. The mechanical properties of the joints under various conditions were analyzed based on experimental observations, load–displacement curves, strain responses, and surface temperature distributions of specimens. Moreover, comparisons were made between the deformation behaviors of joints with and without cracks, and finite element analysis (FEA) was employed for corresponding validation. The main conclusions are as follows: Weld cracking significantly affects joint stiffness and remarkably alters the joint damage mode. Reinforcement plates can effectively strengthen members with cracked welds; however, this reinforcement enhances the stiffness of the tension branch region, thereby altering the force transmission mechanism of the joints. This research offers theoretical and experimental insights for the engineering application of K-joint structures.