The outbreak of the 2019 coronavirus disease (COVID-19) has significant effects on stress, emotion and sleep in the general public. The aim of this study was to explore the relationship between perceived stress and emotional symptoms during the first wave of the COVID-19 pandemic in China and to further determine whether insomnia could serve as a mediator in this relationship. A total of 1178 ordinary citizens living in mainland China conducted anonymous online surveys. The 10-item Perceived Stress Scale, the Insomnia Severity Index, the 9-item Patient Health Questionnaire and the 7-item Generalized Anxiety Disorder scale were used to estimate perceived stress, insomnia, depression and anxiety symptoms, respectively. Of the 1171 valid respondents from 132 cities in China, 46.6 % and 33.0 % showed symptoms of depression and anxiety, respectively. Perceived stress and insomnia independently predicted the prevalence of emotional symptoms and were positively correlated with the severity of these emotional symptoms. The mediation analyses further revealed a partial mediation effect of insomnia on the relationship between perceived stress and emotional symptoms during the first wave of the COVID-19 outbreak in China. Our findings can be used to formulate early psychological interventions to improve the mental health of vulnerable groups, specifically those with insomnia, during the COVID-19 pandemic.