Objective To investigate the expression significance of microRNA-155 (miR-155) in peripheral blood of children with Mycoplasma pneumoniae pneumonia (MPP). Methods Children with MPP who admitted to a hospital from December 2016 to June 2019 were selected and divided into common MPP group (CMPP group) and refractory MPP group (RMPP group), according to whether there was extrapulmonary injury, children with RMPP were divided into extrapulmonary injury group and non-extrapulmonary injury group; miR-155, interferon-γ (IFN-γ) and interleukin-4 (IL-4) in peripheral blood of children in different groups were detected and compared. Results There were 136 children with MPP, 80 in CMPP group and 56 in RMPP group; among children with RMPP, there were 25 cases in extrapulmonary injury group and 31 in non-extrapulmonary injury group. Levels of IFN-γ, IL-4 and miR-155 in RMPP group were higher than those in CMPP group, IFN-γ/IL-4 was lower than those in CMPP group, differences were all statistically significant (all P < 0.05); miR-155 level and IFN-γ/IL-4 in RMPP group was negatively correlated (r=-0.307, P=0.021); the area under the receiver operating characteristic (ROC) curve of miR-155 for diagnosis of RMPP was 0.742, and miR-155 was an independent influencing factor for RMPP; compared with the non-extrapulmonary injury group, levels of IL-4 and miR-155 in extrapulmonary injury group increased, and IFN-γ/IL-4 decreased, differences were all statistically significant (all P < 0.05); the level of miR-155 and IFN-γ/IL-4 in extrapulmonary injury group was not correlated (P>0.05); the area under the ROC curve of miR-155 for the diagnosis of RMPP complicated with extrapulmonary injury was 0.724, and miR-155 was an independent risk factor for RMPP complicated with extrapulmonary injury. Conclusion miR-155 is highly expressed in peripheral blood of children with RMPP, which is closely related to T lymphocyte balance, and has certain diagnostic value for the diagnosis of RMPP and the early identification of extrapulmonary injury.