Objective To investigate the relationship between blood Th17/regulatory T cell (Treg) imbalance and myocardial damage in children with mycoplasma pneumonia. Methods The 137 children with mycoplasma pneumonia in Xingtai People′s hospital from March 2020 to March 2022 were selected as the research objects. In addition, 96 children who underwent physical examination in the hospital from March 2020 to March 2022 were selected as the control group. Myocardial damage was determined by creatine kinase isoenzyme (CK-MB) level, imaging examination and ECG. The proportion of Th17 cells and Treg cells in CD4+ T cells was measured by flow cytometry. The levels of lactate dehydrogenase (LDH), creatine kinase (CK) and CK-MB were measured by automatic biochemical analyzer. The occurrence of myocardial damage in children with mycoplasma pneumonia, Th17 cells to CD4+ T cells ratio, Treg cells to CD4+ T cells ratio, the levels of Th17/Treg and myocardial enzymes were observed. The correlation between Th17/Treg and myocardial enzymes were analyzed. Results Among 137 children with mycoplasma pneumonia, 72 cases had myocardial damage, accounting for 52.55%. The levels of serum LDH, CK and CK-MB in myocardial damage group were higher than non-myocardial damage group and control group, and the levels of serum LDH, CK and CK-MB in non-myocardial damage group were higher than control group (P<0.05). The proportion of Th17 cells in CD4+ T cells and Th17/Treg in myocardial damage group were higher than non-cardiac muscle damage group and control group, and the proportion of Treg cells in CD4+ T cells was lower than non-cardiac muscle damage group and control group (P<0.05). Pearson correlation analysis showed that Th17/Treg was linearly and positively correlated with LDH, CK and CK-MB.Conclusion The levels of myocardial enzymes LDH, CK and CK-MB in children with mycoplasma pneumonia complicated with myocardial damage are significantly increased, and the ratio of Th17/Treg cells is unbalanced, which is linearly positively correlated with LDH, CK and CK-MB