To promote the resource utilization of sericulture byproducts, this study investigated the effects of initial particle size of mulberry branches on the composting process and compost maturity by mixing mulberry branches and silkworm faeces in equal mass. The results showed that although smaller particle sizes (<1 cm) could better increase the compost piles temperature and maintain it at a higher level, while also effectively promoting the degradation of organic carbon, piles with larger particle sizes (>1 cm) had appropriate bulk densities, which significantly promoted the degradation of hemicellulose and cellulose during the composting process. These larger-particle-size piles also exhibited a greater decrease in C/N ratio, indicating more thorough decomposition of organic matter and higher maturity efficiency. After composting, group C (particle size>2 cm) had the highest contents humus, available nitrogen, phosphorus, potassium, and total phosphorus and potassium, resulting in the best compost quality. Overall, controlling the mulberry branch particle sizes at 2-7 cm in compost piles made from a mixture of mulberry branches and silkworm faeces (at equal mass) facilitates the degradation of (hemi) cellulose and improves of compost quality.