Objective To analyze the clinical characteristics and laboratory findings of 348 children with infectious mononucleosis (IM). Methods Clinical features, complications and laboratory findings of children hospitalized with IM from January 2019 to February 2022 were included. The clinical characteristics, complications and laboratory indicators were retrospectively analyzed. Results A total of 348 children with IM were included, 185 male (53.2 %) and 163 female (46.8 %), aged 1-15 years old, mainly 4-6 years old (preschool group) (49.1 %) ; The season of onset was mainly in summer, with a high incidence in June-August. The main clinical symptoms were fever in 289 cases (83.0 %), eyelid edema in 192 cases (55.2 %), nasal obstruction 182 cases (52.3 %). Compared with the ≤3 years old and 4-6 years old groups, the >6 years old group had a lower proportion of fever and a higher proportion of pharyngitis (P<0.05). The incidence of angina and lymphadenopathy in school-age group was significantly higher than ≤3 years old group (infant group) (P<0.05), and the incidence of hepatosplenomegaly in preschool group was significantly higher than the other two groups (P<0.05). The symptom of nasal obstruction in infant group was significantly higher than other two groups (P<0.05). The differences in lymphocyte counts, CD4⁺/CD3⁺, CD8⁺/CD3⁺, CD4⁺/CD8⁺, glutamate aminotransferase, glutamyl transpeptidase, and lactate dehydrogenase between the ≤3, 4-6, and >6 year old groups were statistically significant (P<0.05). Complications in children with IM were predominantly liver injury in 133 cases (38.2 %), followed by neutropenia (53 cases, 15.22 %), and sepsis (27 cases, 7.75 %). Compared to the non-hepatic injury group, the hepatic injury group had a lower proportion of males, older age, higher lymphocyte counts, and lower neutrophil-lymphocyte ratios (NLR) and monocyte-lymphocyte ratios (MLR), with a statistically significant difference (P<0.05). Conclusion IM in children has a high incidence, predominantly in preschool children, with variable and atypical early clinical manifestations and liver injury as the most common complication. Gender, age, lymphocyte count, NLR and MLR may be associated with liver injury.