Abstract: Objectives　To analyze the clinical characteristics and significance of Mycoplasma pneumoniae pneumonia (MPP) combined with secondary thrombocytosis in infants and young children. Methods　Clinical features, laboratory and imaging data of the infants and young children with MPP were collected, and compared between the two groups of children with and without secondary thrombocytosis. Results　Sixty-seven (67) infants and young children with secondary thrombocytosis (28 males and 39 females, with onset at 2 years and 2 months after birth in average) and 269 infants and young children with normal platelet counts (138 males and 131 females, at 2 years and 3 months after birth in average) were included. Significant longer durations of hospitalization（P =0.018） and fever（P =0.000）, higher temperature peak（P =0.000）, as well as higher morbidity of refractory MPP（P =0.001） and more complications（P =0.000）were observed in the group of MPP with secondary thrombocytosis. Moreover, the laboratory data of white blood cell count（P=0.000）, C-reactive protein（P=0.000），procalcitonin （P=0.000）, erythrocyte sedimentation rate（P=0.000）and higher morbidity of multiple pathogen infection（P =0.033） were observed between the two groups. Conclusions　More severe clinical manifestations and higher complication morbidity could be observed in the group of  MPP combined with secondary thrombocytosis, implicating the value of platelet count  in clinical assessment of MPP in infants and young children at early stage.