Abstract: An underground grain silo with assembled steel plate-concrete composite is characterized by land and energy saving, carbon reduction, green environmental protection, airtight and low temperature. The connection joint is one of the most critical parts of the assembled structure. Structural form and mechanical properties have restricted the development, popularization, and application of underground assembled composite structures. All-welded joints are currently used in underground silos, leading to the weld challenging with numerous weld seams. In this study, a type of bolt-weld vertical joint was proposed for the assembled composite underground silo with steel plate-concrete composite. The bolts were also used to replace the portion of the weld. The length of the weld was then reduced to easily position the members. Both bolts and welds were used to connect the assembled parts of concrete, internal steel plate, external waterproof steel plate, and internal waterproof steel plate. Two specimens of bolt-weld vertical joints were designed: A single-sided three- and five-bolt specimen. Two-point symmetric loading tests were carried out to determine the bending performance of steel plate-concrete composite plates. A systematic analysis was made of the failure pattern and bearing capacity of specimens. The results indicate that the single-sided three- and five-bolt specimens shared similar bearing capacities of 1 998 and 2 053 kN, respectively. The load-displacement curves showed that the single-sided five-bolt specimen was much more susceptible to the brittle damage, whereas, the single-sided three-bolt specimen exhibited better ductility after failure. Therefore, a three-bolt joint was recommended to design the joint in a practical project. A finite element model of assembled underground grain silo joint was then established to derive the bearing capacity. The relative error of the overall average on deflection finite element and test strain for SC-3 was 4.17%, indicating agreement with the excellent numerical and experimental one. A parameter study was conducted to establish finite element models with various design variables. A systematic investigation was made to clarify the influence of concrete strength, the thickness of internal waterproof steel plate, external waterproof steel plate, and internal steel plate on the ultimate bearing capacity and peak displacement of the structure. The regression coefficients of concrete strength, internal waterproof steel plate thickness, external waterproof steel plate thickness, and internal steel plate thickness to peak load were 0.993, -0.003, -0.012, and 0.056, respectively. The bending bearing capacity of the joint depended significantly on the strength of the concrete, while there was little effect on the thickness of the steel plate. The increasing concrete strength more effectively improved the bearing capacity of the joint in the practical project. The higher concrete strength may result in the low ductility of the specimen. Therefore, it is recommended to select the optimal concrete strength. This finding can provide reliable scientific information to design the connection joint of the assembled composite underground silo with steel plate-concrete.