Open metal site(OMS)seated in a luminescent lanthanide(Ln)metal center offers an opportunity for rationally tuning the spectroscopic behavior of lanthanide-organic frameworks aiming for a wide range of sensing applications.However,given the spherical nature of common coordination geometries of trivalent lanthanides and the generally strong Ln-O bonds,the lan-thanide based OMS is rarely reported and difficult to be functionalized.We report here a unique europium-organic framework containing abundant quasi-OMS that is protected by an abnormal weak Eu-O bond.These quasi-OMSs offer reversible direct binding sites for water molecules probed by X-ray crystallography,leading to sensitive,visible,and ratiometric luminescent sensing toward humidity and water content in organic solvents.The specific recognition of water based on quasi-luminescent-OMSs gives rise to a superior water detection limit down to 0.0003％v/v,which is one order of magnitude lower than that of Karl Fischer method.