In this work, we found that 14-deoxy-11,12-didehydroandrographolide (2), a derivative of andrographolide (AP, 1), had greatly reduced cytotoxicity compared with AP and exhibited moderate anti-osteoclastogenesis activity. Thirty compounds were synthesized by introducing anti-osteoporosis chemotypes at C-19 of 2. Six of them exhibited stronger inhibition of osteoclastogenesis than AP. Of note, compound 12g displayed the most potent activity with IC value of 0.35 μM. The expression levels of osteoclast-specific genes such as TRAcP, CTSK, NFATc1, and MMP-9 were also decreased by 12g treatment. Furthermore, Western blot and immunofluorescence analyses demonstrated that compound 12g inhibited osteoclast differentiation through downregulation of RANKL-induced NF-κB signaling pathway. In an ovariectomized (OVX) female mice model, compound 12g significantly ameliorated bone loss. Therefore, compound 12g exhibited promising in vivo efficacy and low toxicity, indicating its therapeutic potential for the treatment of osteoporosis.