Lysine demethylase 6B (KDM6B) plays a role in regulating osteoblast differentiation and fetal bone ossification. Nevertheless, its involvement in regulating postnatal bone homeostasis and bone mass accrual remains unclear. In this study, we generated mice lacking Kdm6b gene specifically in mesenchyme and osteoprogenitor cells using a conditional strategy. The adult mice of both mutant strains had decreased cancellous bone mass. The absence of Kdm6b in mesenchyme led to decreased numbers of osteoblasts and osteoclasts, increased marrow adipocytes, as well as repressed bone formation and resorption. Additionally, Kdm6b-deficient bone marrow stromal cells (BMSCs) displayed impaired osteogenic differentiation and exerted an inhibitory effect on osteoclastogenesis. RNA-seq combined with gene expression analysis uncovered downregulation of collagen triple helix repeat containing 1 (CTHRC1) and a lower receptor activator of nuclear factor κB ligand (RANKL)/osteoprotegerin (OPG) ratio in BMSCs of the mutant mice. Further mechanistic explorations demonstrated that KDM6B epigenetically upregulated CTHRC1 expression by removing the repressive H3K27me3 mark from its promoter, thereby triggering PKCδ/MAPKs signaling to facilitate osteoblast differentiation. CTHRC1 was able to mitigate the dysregulated osteogenic and adipogenic differentiation induced by Kdm6b deficiency. This study provides evidence that KDM6B regulates postnatal bone homeostasis through balancing osteoblast and osteoclast differentiation. Given its predominant promotion of osteoblastic bone formation over osteoclastic bone resorption, KDM6B tends to promote postnatal bone mass accrual.