Introduction: The study aimed to investigate the prevalence of overweight and obesity among Chinese colorectal cancer survivors, and how they gain weight during treatment. Methods: A sequential mixed method study was conducted. Pearson correlation analysis, univariate analysis and a multivariable-adjusted binary Logistic regression model were constructed to explore risk factors. Semi-structured in-depth interviews were conducted, and content analysis was employed for qualitative data analysis. Results: There were 30.8% overweight and 4.2% obese colorectal cancer survivors. Gender (b = -1.10), caregiver (b = -0.76), dietary knowledge (b = -0.13, anxiety (b = 0.14), depression (b = -0.15), duration of illness (b = 0.95), and tumor staging (b = -1.25) were significantly associated with overweight and obesity. A total of four categories, six themes, and 12 codes were identified, revealing a "teachable moment" for lifestyle modification. Discussion: One-third of the patients were overweight and obese. Being diagnosed was a "teachable moment" for lifestyle modification, but survivors showed limited knowledge and incorrect weight management goal.