Mechanical stimuli are crucial in regulating normal osteoblast function to maintain bone homeostasis. Decreased osteoblast proliferation is one of the main causes of osteoporosis induced by mechanical unloading. However, the underlying mechanism remains unclear. Cyclin-dependent kinase 12 (CDK12), a member of the CDK family, plays a key role in regulating cell proliferation. In this study, we aimed to investigate the role of CDK12 in osteoblast proliferation, and its function and underlying mechanism in decreased osteoblast proliferation caused by mechanical unloading.