Background Although physical activity (PA) and sedentary time in cancer survivors (CSs) were associated with health-related quality of life (HRQOL), it was not clear whether their associations were similar among CSs with different number of comorbid chronic diseases (CCDs). This study aimed to investigate the associations between PA, sedentary time and HRQOL in CSs with different number of CCDs. Methods A cross-sectional study was conducted among 1546 CSs between June and September 2018 in

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... hai, China. Data were collected with a self-reported questionnaire including sociodemographic characteristics, CCDs, PA, sedentary time and HRQOL. International Physical Activity Questionnaire and Cancer Quality of Life Questionnaire-Core30 were respectively used to measure PA and HRQOL of CSs. Associations of PA and sedentary time with HRQOL among CSs with different number of CCDs were evaluated by using logistic regression, adjusted for confounding factors. Results About seventy-five percent CSs had at least one CCD. Approximately three fifths CSs had high PA level and < 4 h/day sedentary time. Moderate PA level and high PA level were shown to be associated with better HRQOL among all participants. In CSs with ≤ 2 CCDs, high PA level was significantly associated with higher scores of physical function and lower scores of nausea and vomiting, appetite loss. However, there was a positive association between high PA level and constipation score among CSs with ≥ 3 CCDs. CSs with shorter sedentary time had better HRQOL in those with CCDs. Conclusions High PA level and long sedentary time have significant association with worse HRQOL of CSs with ≥ 3 CCDs, while high PA level is positively associated with HRQOL in CSs with ≤ 2 CCDs. Our findings may support further studies of the causal association between PA, sedentary times and HRQOL to provide targeted proposal to improve the HRQOL of CSs according to their number of CCDs.