Although keratin can effectively repair hair fibres and enhance their moisture content and flexibility, it has a relatively low affinity for hair. In this study, the effects of transglutaminase (TGase)—commonly used to catalyse crosslinking of proteins or amino acids—in crosslinking serine and hydrolysed wool keratin to repair damaged hair and protect healthy hair were studied. Treatment with a repair solution containing hydrolysed wool keratin, serine, and TGase improved the physical and chemical properties of damaged hair samples. The alkali solubility of damaged hair samples decreased by 50.53%, fracture stress increased from 1.031 to 1.806 N, and fracture strain increased from 9.51 to 19.88 mm. Fourier transform infrared spectroscopy and X-ray analysis showed that amide bonds increased in damaged hair samples treated with the repair solution and hair crystallinity increased. Differential scanning calorimetry showed that the repair solution improved the thermal stability of damaged hair. After five cycles of washing, the effects of the repair solution were still apparent in damaged hair samples. The enzymatic solution had stronger repair effects than general hair care products and reduced water loss rates in damaged hair samples; repaired hair samples were also softer and brighter. The repair solution was effective in protecting healthy hair samples against chemical damage. The materials used to prepare the repair solution are all bio-based, and treatment with this product is safer and longer lasting.