As an interdisciplinary technology with the integration of mechanical engineering, science of control, biomechanics and medicine, lower limb exoskeleton system has emerged as a pivotal solution to address lower limb motor impairments caused by population aging, stroke and cerebral palsy. This paper provides a systematic review of the core technological advancements in lower limb exoskeleton system, involving in mechanical design, strategy of control, biomechanical modeling and simulation, as well as clinical application outcomes. For mechanical design, diverse drive structures have been developed involving in active, passive and semi-active configurations to cater to different user groups and usage scenarios. Strategy of control have evolved from traditional hierarchical control to AI-driven adaptive and intelligent control and integrated into biological signals such as electromyography (EMG) and electroencephalography (EEG) to achieve human-machine collaboration. Biomechanical models (including conceptual and detailed representations) offer theoretical support for system design and control by elucidating human motion mechanics. Clinical application have demonstrated the efficacy of these systems in restoring gait and motor function in patients with stroke, cerebral palsy and war-related injuries. Additionally, this paper identifies key challenges in current lower limb exoskeleton system including insufficient customization, suboptimal human-machine interaction compliance and a lack of standardized clinical protocols. Furthermore, it outlines the future development of customized technologies, AI-adaptive control and multimodal interaction so as to guide the optimization and clinical translation of lower limb exoskeleton systems.