Objective To explore the relationship of the area under the mycophenolic acid concentration-time curve (MPA AUC) with disease activity, disease recurrence and adverse drug reactions of mycophenolate mofetil (MMF) in the treatment of childhood systemic lupus erythematosus (SLE). Methods The clinical data of SLE children who underwent MMF blood concentration test from January 2017 to June 2019 were retrospectively analyzed. The treatment regimens for the children in the last half year were hormone combined with MMF and hydroxychloroquine, and the dose of MMF was 20 - 40 mg/(kg·d). According to SLE disease activity score (SLEDAI), recurrence and adverse reactions and the levels of MPA AUC were compared among the groups. According to the level of MPA AUC, the patients were divided into three groups, and the disease activity and adverse reactions among the three groups were compared. Results A total of 81 ( 20 boys and 61 girls) children were enrolled and the median age was 13 . 1 ( 11 . 0 - 14 . 4 ) years. According to the 3 month-SLEDAI, 67 children were included in the inactive group and 14 were included in the active group. Compared with the inactive group, age at onset was younger, albumin and creatinine levels were lower and eGFR was higher in active group, and the differences were statistically significant (all P< 0 . 05 ). The MPA AUC0 - 12 h at 3 -month follow-up and 6 -month follow-up in the inactive group were higher than those in the active group, and the difference was statistically significant (P< 0 . 05 ). There were statistically significant differences in SLEDAI among the low, medium and high AUC groups at 3 month and 6 month follow-up, and the SLEDAI in low AUC group were the highest. The level of MPA AUC in the relapsed group was lower than that of the non-relapsed group, and the difference was statistically significant (P= 0. 001 ). The cut-off point of MPAAUC was 43. 03 mg /(h·L) when the ROC curve was used to predict the recurrence. The AUC was 0 . 89 (P= 0 . 001 ), the sensitivity was 100% and the specificity was 69 . 8%. Conclusions? It is important to monitor MPA AUC in the treatment of childhood SLE with MMF. There was a negative correlation between MPA AUC and disease activity. The serum concentration of mycophenolic acid higher than 43 . 03 mg/(hL) can maintain the low activity level of SLE and reduce disease recurrence, but excessive concentration does not improve disease activity.