Abstract: Abstract: The underground granary with the prefabricated steel plate-concrete composite has presented the many advantages of energy conservation, loss reduction, and high grain quality, particularly for the requirements of low-carbon environmental protection and sustainable development. However, there are only limited reports on the vertical joints in the prefabricated steel plate-concrete composite wall. In this research, a new type of joint was proposed suitable for the precast steel plate-concrete composite wall in an underground granary. Four groups of steel plates were composed of the U-shaped, external/internal waterproof, and trapezoidal load-transfer steel plate. When the external waterproof steel plate of the new joint was used as the flange plate of the I-steel pile in the foundation pit supporting structure, the steel pile was assembled and connected with the precast block of the underground granary wall, further to realize the integrated construction of foundation pit and granary wall. Three types of precast steel plate-concrete composite specimens were designed to fabricate, including a new joint, non-trapezoidal load-transfer steel plate, and no joint. A two-point symmetrical loading flexural test was conducted to investigate the flexural performance of the precast steel plate-concrete composites in the elastic-plastic stage. An analysis was made on the failure modes, internal forces, and deformation behavior in each specimen under loads. The results indicated that there was a different development of crack and deformation in each specimen under the loading. Most cracks in the two specimens with joints appeared in the compression zone near the joints and the loading zone. By contrast, most cracks with non-joint appeared in the tension zone at the connection between the internal steel plate and concrete. There was the smallest deformation of the specimen with the new joint, and the largest that with no joint. The displacement and strain of each specimen increased with the increasing bending moment. Overall, the upper part of the specimen was in a compression state, whereas, the lower part was in a tension state. Specifically, the inflexibility of the new joint specimen increased significantly, where the flexural capacity increased by 15%, compared with the non-joint specimen. The stiffness and bearing capacity of prefabricated specimens were improved via the increasing concrete strength and the appropriate material parameters of the steel plate. Furthermore, the inflexibility of the new joint specimen increased remarkably, where the flexural capacity increased by 17%, and the yield load increased by 29%, indicating that the trapezoidal load-transfer steel plate effectively improved the joint performance, compared with the non-load-transfer steel plate joint. The mid-span tensile strains of the trapezoidal load-transfer and internal waterproof steel plate increased linearly with the increment of bending moment, where the tensile deformation was consistent, indicating that both of them played a tensile role in the new joint. The findings can provide a strong reference to design the flexural performance of prefabricated underground granaries and similar underground structures.