Abstract: UV-B (280–320 nm) sunscreening is crucial for lives on Earth. Examining the role of surrounding of UV-B screening molecules can help us better understand UV-B absorption. Water is ubiquitous in cells as the solvent, but its impacts on UV-B absorption of sunscreen agents are underexplored. Herein, we report a first-principle study on UV-B absorption of sinapate malate (SM) and relevant molecular species, with a focus on the solvent effect of water. The capability of UV-B screening by anions of SM, the proposed species for actual sunscreening, is shaped by water. Both the implicit water providing the dielectric environment of solvation and the explicit water molecules forming hydrogen bonding to SM anion can appreciably alter the nature of transition orbitals responsible to the UV-B absorption of the anion. Finally, we find the molecular dipole moment of an organic UV-B screening agent can be an indicator of its UV-B screening sensitivity to the surrounding water. Our work may serve as a starting point of developing new water-soluble UV-B screening agent.