The complex preservation conditions make underwater cultural relics, especially organic ones—more fragile than terrestrial relics. Bamboo slips, the main carriers of ancient Chinese characters, are often found underwater. They have high cultural and historical values, but their mechanical strength is low, making them difficult to extract. Moreover, if their arrangement is disrupted during the extraction process, the subsequent restoration will be harder. In this study, veratraldehyde, a new underwater temporary solid material, was used to conduct in-situ reinforcement and extraction experiments on organic cultural relics. Based on these experiments, the researchers designed a three-layer wrapped protective frame and proposed the recovery method of dissolution and effective residue detection of nuclear magnetic resonance (NMR). The experimental results showed that this method can nearly preserve the original state of fragile bamboo slips underwater and extract them out of the water safely. Also, the solidification process is reversible and without residues. It is a safe and reliable underwater technology for solidifying extraction of cultural relics.