Malnutrition is prevalent among cancer patients, and oral nutritional supplements (ONS) are an important intervention. However, poor adherence to ONS negatively impacts therapeutic outcomes. Previous studies have predominantly employed quantitative methods to investigate factors influencing adherence, with a limited in-depth understanding of patients' subjective experiences and multidimensional mechanisms.This study adopted a descriptive qualitative design based on the social cognitive theory (SCT) framework. Semi-structured interviews were conducted with 14 cancer patients who had experience using ONS. Thematic analysis was used to identify core themes influencing adherence.Three major dimensions affecting adherence were identified: (1) Personal Factors: Misconceptions about ONS, symptoms related to intake, and insufficient self-efficacy; (2) Behavioral Factors: Variations in health information management, diverse coping strategies, and fluctuating adherence across treatment phases; (3) Environmental Factors: Product incompatibility, financial burden and inadequate policy coverage, and lack of continuity of care outside the hospital.Adherence to ONS among cancer patients is influenced by the interplay of individual cognitive and emotional factors, dynamic behavioral patterns, and external environmental conditions. Improving adherence requires a multidimensional, integrated intervention strategy that incorporates all three elements of SCT, including education to clarify misconceptions, skills training to enhance self-efficacy, optimization of product experience, alleviation of financial burden, and strengthening of systemic support.© 2025. The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature.