Objective: To understand the prevalence of insufficient physical activity among adults aged ≥18 years in China and to explore the influencing factors. Methods: The China Chronic Disease and Risk Factor Surveillance was conducted in 298 counties/districts in China in 2018, covering 31 provinces (autonomous regions, municipalities). A multi-stage stratified cluster random sampling method was used to select permanent residents aged ≥18 years. A questionnaire including Global Physical Activity Questionnaire was used to collect information about the participants' demographic characteristics and physical activity through face-to-face interview. A total of 183 769 participants completed the survey. After complex data weighting, the prevalence of insufficient physical activity, occupation, transportation, and leisure-time physical activity time was analyzed. Multivariate logistic regression models were used to analyze the influencing factors related to insufficient physical activity. Results: The prevalence of insufficient physical activity among adults aged ≥18 years was 22.3% (95%CI: 20.9%-23.7%) in China in 2018, with males [24.4% (95%CI: 23.0%-25.8%)] significantly higher than females [20.2% (95%CI: 18.6%-21.8%)]. Adults aged 70 years and above [28.4% (95%CI: 26.9%-29.9%)] were significantly higher than adults in other age groups, followed by adults aged 18-29 years [26.4% (95%CI: 24.4%- 28.3%)] and 30-39 years [23.4% (95%CI: 21.5%-25.3%)], and tended to increase with increasing education and total sedentary behavior time (P<0.001). The weekly occupation, transportation, and leisure-time physical activity time appeared 958.6 (95%CI: 911.4-1 005.8) minutes, 234.5 (95%CI: 224.7- 244.2) minutes, and 88.6 (95%CI: 83.5-93.7) minutes, respectively. Multivariate logistic regression analysis showed that males, adults living in rural areas or northern China, ≥70 years, with junior high school education, an annual household income per capita <6 000 yuan and institutional/clerical/ technical occupation and longer total sedentary behavior time were related to a higher prevalence of insufficient physical activity. Conclusions: In China, over one-fifth of the adults had lower physical activity levels. Adults who are male, young adults, more educated, institutional/clerical/technical occupation, and with more extended total sedentary behavior are the populations that need to be focused on to promote physical activity-related health.