Real-time and in situ detection of aqueous solution is essential for bioanalysis and chemical reactions. However, it is extremely challenging for infrared microscopic measurement because of the large background of water absorption. Here, we proposed a wideband-tunable graphene plasmonic infrared biosensor to detect biomolecules in an aqueous environment, employing attenuated total reflection in an Otto prism configuration and tightly confined plasmons in graphene nanoribbons. Benefiting from the graphene plasmonic electric field enhancement, such a biosensor is able to identify the molecular chemical fingerprints without the interference of water absorption. As a proof of concept, the recombinant protein AG and goat anti-mouse immunoglobulin G (IgG) are used as the sensing analytes, of which the vibrational modes (1669 and 1532 cm-1) are very close to the OH-bending mode of water (1640 cm-1). Simulation results show that the fingerprints of protein molecules in the water environment can be selectively enhanced. Therefore, the water absorption is successfully suppressed so that two protein modes can be resolved by sweeping graphene Fermi energy in a wide waveband. By further optimizing the incident angle and graphene mobility to improve the mode energy of graphene plasmons, maximum enhancement factors of 112 and 130 can be achieved for amide I and II bands. Our work provides an effective approach for the highly sensitive and selective in situ identification of aqueous-phase molecular fingerprints in fields of healthcare, food safety, and biochemical sensing.