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Constraining the lithospheric rheology of Tibetan Plateau is important for the physical understanding of its tectonics. Siling Co, the largest high‐altitude lake in the world, has experienced a rapid water level increase in the 2000s. The resulting loading changes stimulate the viscoelastic response of the lower crust, giving access to study the lithospheric rheology in central Tibet. Here, we derive a clear subsidence signal around the lake with peak velocity of 4 mm/yr from two tracks of Sentinel‐1 InSAR images acquired from 2017 to 2022. Our viscoelastic modeling suggests a ductile layer 15 km beneath the surface with a decadal‐scale, steady‐state viscosity of 1–4 × 1019 Pa s. This value is consistent with the viscosity inferred from millennium‐scale shoreline changes, but is about 10 times higher than the viscosities derived from the deformation surrounding Siling Co before 2011, highlighting the role of transient viscosity in controlling the surface deformation. Plain Language Summary: The change of surface loading will cause the response of earth lithosphere, including instantaneous, concentrated elastic and delayed, wide‐range viscoelastic deformation at the surface. With the change of climate, the amount of water in many lakes in the Tibetan Plateau has changed. The resulted water loading changes can cause surface deformation that can be measured by space‐borne radar images. Using the Interferometric Synthetic Aperture Radar (InSAR), we measure the deformation around the Siling Co, the largest high‐altitude lake in the world, with the peak subsidence velocity of up to 4 mm/yr. According to numerical modeling, we find that the deformation mainly comes from the viscoelastic response of the ductile layer 15 km beneath the surface. The inferred viscosity of 1–4 × 1019 Pa s is consistent with the value derived from millennium‐scale shoreline changes. Our research links the surface displacement with the strengths of the deep earth, providing a direct constraint for geodynamics models. Key Points: We observe subsidence around the Siling Co from Sentinel‐1 InSAR images acquired between 2017 and 2022 with a peak rate of 4 mm/yrA ductile layer below 15 km with a viscosity of 1–4 × 1019 Pa s can explain the observed subsidence around the Siling CoThis decadal‐scale viscosity is consistent with the millennial‐scale value derived from paleoshoreline changes around the Siling Co [ABSTRACT FROM AUTHOR]