Handmade papers, as carriers of paper-based cultural relics, have played a crucial role in the development of human culture, knowledge, and civilization. Understanding the intricate relationship between the structural properties and degradation mechanisms of handmade papers is essential for the conservation of historical documents. In this work, an artificial dry-heat accelerated ageing method was used to investigate the interplay between the mechanical properties of paper, the degree of polymerization (DP) of cellulose, chemical composition, hydrogen bond strength, crystallinity, and degree of hornification. The results show that the mechanical properties of handmade bamboo paper exhibited a first plateau region, a rapid decline region, and sometimes a second plateau region. A critical point in the mechanical properties of the paper occurs when the cellulose DP decreases to a range of 400-600, signifying a shift from the initial plateau to a sharp decline phase. The strengthening of intermolecular hydrogen bonds and the hornification process help counteract the embrittlement of fibers caused by cellulose chain scission due to DP reduction. A secondary plateau emerges when the DP is smaller than 400, cellulose degradation is slow, and the component content, hydrogen bond strength, crystallinity, and degree of hornification reach a secondary plateau.