Background Sibling relationships play a critical role in shaping anxiety symptoms in firstborn children. Anxiety symptoms often originate in early childhood and can persist into adolescence and adulthood. However， there is insufficient research on anxiety symptoms in preschool children， especially firstborn preschool children.Objective To explore the influencing factors of anxiety symptoms among firstborn preschool children， so as to provide references for the intervention of anxiety symptom for children in families with multiple children.Methods From October to December 2021， a total of 8 449 children from 234 kindergartens in Longhua District of Shenzhen were included using a cluster sampling method. Sibling Inventory of Behavior （SIB） and Spence Preschool Anxiety Scale （SPAS） were used to investigate. Logistic regression analysis was used to identify influencing factors of anxiety symptoms in firstborn preschool children.Results A total of 8 419 （99.64%） valid questionnaires were collected. Anxiety symptoms were detected in 344（4.09%） firstborn preschool children. Statistically significant differences were observed between anxiety group and non-anxiety group in terms of household registration， monthly family income， maternal age， maternal education level， paternal education level， family living conditions and whether they are left-behind children （χ2/t=9.906， 33.490， 5.136， 13.485， 9.690， 17.332， 21.975， P<0.05 or 0.01）. Compared with non-anxiety group， children in the anxiety group scored higher on the SIB dimensions of rivalry， aggression and avoidance （t=165.322， 74.471， 286.419， P<0.01）， and lower on companionship， empathy and teaching （t=59.133， 42.417， 39.112， P<0.01）. Risk factors for anxiety symptoms in firstborn preschool children included left-behind children， as well as negative sibling relationships characterized by rivalry and avoidance （OR=1.195， 1.143， 1.260， P<0.05 or 0.01）.Conclusion Firstborn preschool children who are left-behind are more susceptible to anxiety symptoms. Negative sibling relationships， characterized by competition and avoidance， may also contribute to the emergence of anxiety symptoms in firstborn preschool children.