Adjacent segment degeneration (ASDeg) and adjacent segment disease (ASD) after cervical fusion are the core issues affecting long-term outcomes. The imaging incidence is 16% to 96%, and the incidence of symptomatic ASD is 1.8% to 36%. About 40% require secondary surgery. The core mechanism is that fusion causes an abnormal increase in biomechanical load on adjacent segments (a 73.2% increase in proximal disc pressure during flexion) and accelerates degeneration. Risk factors include multiple dimensions such as preoperative (advanced age, spinal stenosis, multi-level degeneration), intraoperative (multi-level fusion, improper plate positioning) and postoperative (loss of cervical curvature). The diagnosis of ASD requires combining medical history, nerve compression symptoms (neck pain, limb numbness and weakness) and imaging findings (intervertebral space stenosis, osteophytes, spinal canal compression). Treatment needs to be individualized: For mild cases, surgery should be considered for those who fail to respond to non-surgical treatment or have obvious nerve compression. Revision of anterior cervical discectomy and decompression fusion (ACDF) is still the mainstream, but the risk of secondary surgical complications is high; zero-profile fusion cages can significantly reduce the incidence of dysphagia (0% versus 5.2%); posterior laminoplasty is suitable for multi-level spinal cord compression; cervical disc arthroplasty (CDA) preserves range of motion and may delay adjacent segment degeneration. Posterior spinal endoscopic technology (discectomy/spinal canal decompression/nerve root canal decompression) is an emerging treatment direction because it avoids the risk of anterior scarring, has the advantages of minimally invasive, rapid recovery, and little impact on stability. However, more research is needed to verify its long-term efficacy. In the future, it is necessary to strengthen the formulation of evidence-based guidelines and technological innovation to optimize prevention and control strategies.