To investigate the factors influencing the colonoscopy screening behavior of first-degree relatives of colorectal cancer patients and to provide a basis for formulating screening intervention strategies. In this study, 15 first-degree relatives of colorectal cancer patients in the Department of Medical Oncology of a tertiary hospital in Baoding City from May to July 2024 were selected as the research subjects, and face-to-face semi-structured interviews were conducted. The theme was analyzed and summarized based on the theory of planned behavior and Colaizzi's 7-step analysis method. Results of this study identified 12 themes from 3 aspects. For behavioral attitudes, 4 themes emerged: (1) Negative screening emotions caused by psychological stress, (2) Lack of awareness of the importance of screening due to cognitive biases, (3) Screening behavior caused by fatalistic views is systematically underestimated, and (4) Affirm the value of early screening. For subjective norms (The impact of external information on screening behavior), 4 themes emerged: (1) Family support, (2) Advice from a healthcare professional, (3) information support from online media, (4) Personal experience and suggestions from friends. For perceived behavior control, 4 themes emerged: (1) Unbearable pain and embarrassment, (2) Busyness of life and work, (3) Medical treatment process and transportation convenience, and (4) Screening costs. The colonoscopy screening behavior of first-degree relatives of colorectal cancer patients is affected by behavioral attitudes, subjective norms (The impact of external information on screening behavior), and perceived behavior control. Clinical medical staff should correct their behavioral cognitive biases from the perspective of first-degree relatives, use positive belief factors to avoid wrong cognition, pay attention to a variety of sources of support, stimulate the self-efficacy of first-degree relatives, and create a suitable environment for colonoscopy screening, to promote the change of colonoscopy screening behavior of first-degree relatives.