Small collagen peptides are associated with various benefits, such as bone and skin health. However, preparation of small collagen peptides from terrestrial vertebrate origins remains a challenge. Here, we show that pretreatment with formic acid enhanced the production of small collagen peptides from spent hen skin. After pretreatment with formic acid, the percentage of small peptides below 2 kDa increased to 48.92% and 43.34% from 33.79% and 36.32% for heat-soluble collagen (HSC) and pepsin-soluble collagen (PSC), respectively. Pretreatment with formic acid degraded telopeptides and released the cross-links (pyrrole and pyridinoline), which made hen collagen more susceptible to papain hydrolysis. LC-MS/MS results revealed that none of the peptides identified from HSC-FA (formic acid)-Papain and PSC-FA-Papain were derived from cross-linked telopeptides. These results demonstrated that formic acid assisted the hydrolysis of highly cross-linked collagen of spent hens, and it might also be used to produce small collagen peptides from other aged, vertebrate collagens.