Objective To study the correlation of Mycoplasma pneumoniae DNA (MP-DNA) replication level in throat swab and bronchoalveolar lavage fluid (BALF) with disease severity in children with severe Mycoplasma pneumoniae pneumonia (SMPP). Methods A total of 44 children with SMPP who underwent bronchoalveolar lavage were enrolled as subjects. The serum levels of cytokines and MP-DNA replication times in throat swab were measured in the acute stage and the recovery stage, and the levels of interleukin (IL)-8 and MP-DNA replication times in BALF were measured in the acute stage. According to whether mechanical ventilation was needed for respiratory failure, the children were divided into a mechanical ventilation group (n=19) and a non-mechanical ventilation group (n=25), and the two groups were compared in MP-DNA replication times in BALF. Results For the children with SMPP, serum levels of C-reactive protein, erythrocyte sedimentation rate, lactate dehydrogenase, IL-1, IL-6, IL-8, and IL-18 in the acute stage were significantly higher than those in the recovery stage (P < 0.05). In the acute stage, MP-DNA replication times in throat swab were positively correlated with those in BALF (r=0.613, P < 0.05), and MP-DNA replication times in BALF were positively correlated with IL-18 levels in peripheral blood and BALF (r=0.613 and 0.41 respectively, P < 0.05). Compared with the non-mechanical ventilation group, the mechanical ventilation group had significantly higher MP-DNA replication times in BALF, a significantly longer duration of systemic hormone treatment, significantly higher serum levels of lactate dehydrogenase and IL-18, and significantly higher white blood cell count and IL-18 level in BALF (P < 0.05). Conclusions In children with SMPP, MP-DNA replication level in throat swab and BALF can be used as a reference index for the assessment of disease severity.