Abstract: In order to identify the rationality of the engineering design and guarantee the safety of operation, a field waterfilling test is carried out for the first 15×10⁴ m³ extra-large unanchored oil tank on saturated loess foundation. Strain gauges are pasted on oil tank wall and ring wall foundation steel and settlement sensors are buried in the tank foundation to diagnose the tank wall stress, ring wall stress and foundation settlement during water filling. The test results show that, the maximum circumferential stress reaches 284.9 MPa on the tank wall 3.34 m above the tank bottom when the water is filled up to the highest of 22.2 m, and the relationship between the circumferential stress and the water filling level is approximately linear. The maximum ring wall stress reaches 1091 kPa on inner wall 1.0 m from the ring wall top because the upper part of the inner tank wall is endured the pressure stress from the oil tank and the whole inner wall is endured the pressure stress from the inner cushion layer. The settlement deformation of oil tank foundation is mostly plastic deformation, and the deformation can quickly become stable at the end of water-filling test. The entire bottom presents as dish-like settlement. The research results can provide technical basis and design parameter validation for the extra-large oil tank in the design and construction on saturated loess ground.