Objective To understand the clinical characteristics of secondary thrombocythemia in children, and to help clinicians improve their diagnosis and treatment. Methods The clinical data of children with secondary thrombocythemia admitted from January to December 2021 were retrospectively analyzed to summarize the causes of the disease, age and gender distribution, and characteristics of inflammatory factors. Results Three hundred and eighty-six children with secondary thrombocythemia were included, with a mean age of onset of (2.0±0.3) years, and 294 children (76.2%) were<3 years old. The median platelet count was 626.5 (567.8-701.0)×10⁹/L, which peaked at 8.0 (7.0-11.0) days of the disease course and then gradually declined.143 children were regularly monitored for blood counts during the recovery period, and platelets returned to normal at 10.0 (7.0-14.0) days of the disease course. The most common cause of secondary thrombocythemia was infection (94.6%), and the three most common pathogens of infection were Mycoplasma pneumoniae, fungi, and adenovirus. The levels of interleukin (IL)-1β, IL-2 receptor, IL-6, IL-8, IL-10, and tumor necrosis factor (TNF)-α were higher in children with secondary thrombocythemia than in those with normal platelets, and the difference was statistically significant (P<0.05). Conclusion The secondary thrombocythemia is most common in children under 3 years of age, and the most common cause is infection. Platelets can be normalized after treatment for the primary disease, and no inhibition of platelet aggregation and anticoagulation is needed in all children except Kawasaki disease.