The prevalence of and risk factors for uncertainty stress among residents during the COVID-19 pandemic remain unclear. An online cross-sectional survey was conducted to explore and identify the risk factors for high perceived uncertainty stress among the general public in China during the COVID-19 outbreak. Information about the respondents' socioeconomic characteristics, knowledge of and attitudes towards COVID-19, perceived uncertainty stress, social capital, anxiety, and depressive symptoms was collected and analysed. Among the 1205 respondents, 45.3% (546) reported a high level of uncertainty stress. Multiple linear regression analysis indicated that anxiety (β=3.871,P<0.001) and depression symptoms (β=2.458, P<0.001), family residence (in towns or rural areas) (β=0.947, P<0.001), lack of support for local epidemic control strategies (β=1.253, P<0.001), worry about the pandemic (β=1.191, P<0.001), and symptoms of weakness among family members (β=1.525, P=0.002) were positively associated with perceived uncertainty stress. Cognitive social capital (β=-0.883, P<0.001) and social networks (β=-0.726, P<0.001) were negatively, but social participation (β=0.714, P<0.001) was positively associated with perceived uncertainty stress. Our findings identify factors associated with a higher level of uncertainty stress and should be helpful in the consideration of effective policies and interventions for uncertainty stress during the initial phases of public health emergencies.