Abstract: Large-scale oil storage tanks are mostly unanchored and tapered, so complicated phenomena will emerge on the tanks as the foundation settles under hydraulic pressure, such as the warping of edge plate at the bottom and the stress fluctuation at the shell thickness changing zones. To investigate the level and distribution of stress at the shell and bottom board of large-scale loaded tanks, field stress testing was performed on an external-floating-roof oil storage tank with a capacity of 10×10⁴ m³ (D=80 m, H=21.8 m) during the hydrostatic testing by using the resistance strain measurement technology. Based on the stress measured at the tank shell and bottom board, the levels and distribution characteristics of stress at key parts of storage tank and their causes were analyzed. It is indicated that radial bending stress is dominant at the tank bottom while the liquid level is the highest. Axial bending stress is dominant at the root of tank shell. With the rising of measurement point height, the tank shell suffers swelling deformation under the effect of hydraulic pressure, and the circumferential membrane stress is dominant. The research results can be used as the reference for the operation safety assessment of storage tanks.