Background: The epidemiology of Kawasaki disease (KD) suggests that an infectious agent may be a potential disease trigger in susceptible children. Several studies have shown that the cause of KD may be associated with Mycoplasma pneumoniae (M. pneumoniae) infection. Objectives: We aimed to investigate the relationship between M. pneumoniae infection and coronary artery aneurysm (CAA) in children with KD in China. Methods: From January 2015 to December 2018, a total of 330 children with KD met the inclusion criteria. Relevant data were extracted and analysed. Results: The children were stratified into two groups according to M. pneumoniae infection status. Significant differences were identified in the proportion of patients with fever > 10 days, the occurrence of small CAA, and the average serum sodium, pre-albumin (PA), and albumin levels but not in the occurrence of medium and giant CAA between the two groups. According to binary logistic regression, M. pneumoniae infection (OR: 0.515; 95% CI: 0.309 - 0.860; P = 0.011), serum sodium levels (OR: 0.910; 95% CI: 0.851 - 0.972; P =0.005), and PA (OR: 0.900; 95% CI: 0.854 - 0.949; P ≤ 0.001) levels were independently associated with occurrence of small CAA. Conclusions: We demonstrated that M. pneumoniae infection, serum sodium and PA levels are inversely related to the occurrence of small CAA. These results suggest that M. pneumoniae infection may be associated with a decreased incidence of small CAA. Further large-sample studies are needed.