Breeding miscanthus for biomass production and composition is essential for targeting high-yielding genotypes suited to different end-uses. Our objective was to understand the genetic basis of these traits in M. sinensis, according to different plant ages and environmental conditions. A diploid population was established in two locations according to a staggered-start design, which distinguished the plant age effect from climatic condition effect. An integrated genetic map of 2602 SNP markers distributed across 19 LGs was aligned with the M. sinensis reference genome and spanned 2770 cM. The QTL mapping was based on best linear unbiased predictions estimated across three climatic conditions and at least three ages in both locations. A total of 260 and 283 QTL were related to biomass production and composition traits, respectively. In each location, 40-60% were related to biomass production traits and stable across different climatic conditions and ages and 30% to biomass composition traits. Twelve QTL clusters were established based on either biomass production or composition traits and validated by high genetic correlations between the traits. Sixty-two putative M. sinensis genes, related to the cell wall, were evidenced in the QTL clusters of biomass composition traits and orthologous to those of sorghum and maize. Twelve of them were differentially expressed and belonged to gene families related to the cell wall biosynthesis identified in other miscanthus studies. These stable QTL constitute new insights into marker-assisted selection (MAS) breeding while offering a joint improvement of biomass production and composition traits.