Background <p>Cognitive function and decision-making ability are crucial for participation in advance care planning (ACP). People with different degrees of cognitive impairment face considerable difficulty and exhibit significant heterogeneity in their participation in ACP.</p> Methods <p>Six English electronic databases (Embase, PubMed, Cochrane Library, PsycINFO, CINAHL, and Web of Science) and four Chinese databases (CNKI, CBM, Wanfang, and VIP) were searched from inception to February 18, 2024. The QualSyst tool was used to evaluate the study quality. Thematic synthesis was used to analyze the data.</p> Results <p>Thirty-seven papers were included, comprising 21 qualitative, 9 quantitative, and 7 mixed studies. Factors related to ACP antecedents (sociodemographic characteristics, ACP perception, prior ACP experience, disease-related factors, and socio-cultural context) and ACP attributes (ACP procedural elements and ACP supportive mechanisms) influenced the participation of people with different degrees of cognitive impairment in ACP, but some differences were also found. Difficulty in accepting the disease and decision conflicts were not found to have an effect on mild cognitive impairment. Care network relationship was not found to influence mild to moderate dementia. Relying on surrogate decisions, decision conflicts, ACP data storage and informal social support were not found to influence moderate to severe dementia.</p> Conclusions <p>This study determined common and specific factors influencing the ACP participation of people with different degrees of cognitive impairment. The results could provide evidence for developing targeted interventions for promoting people with different degrees of cognitive impairment to participate in ACP.</p>