Rationale: Post-transplant lymphoproliferative disease (PTLD) is a serious complication after pediatric liver transplantation (pLT), which may lead to death. 18F-FDG PET/CT is rarely considered in PTLD after pLT and lacks clear diagnostic guidelines, especially in the differential diagnosis of non-destructive PTLD. The aim of this study was to find a quantifiable 18F-FDG PET/CT index to identify non-destructive PTLD after pLT. Materials and Methods: This retrospective study collected the data of patients who underwent pLT, postoperative lymph node biopsy, and 18F-FDG PET/CT at Tianjin First Central Hospital from January 2014 to December 2021. Quantitative indexes were established using lymph node morphology and the maximum standardized uptake value (SUVmax). Results: A total of 83 patients met the inclusion criteria and were included in this retrospective study. To distinguish between PTLD-negative cases and non-destructive PTLD cases, according to the receiver operating characteristic curve, [the shortest diameter of the lymph node at the biopsy site (SDL)/the longest diameter of the lymph node at the biopsy site (LDL)]*[SUVmax at the biopsy site (SUVmaxBio)/SUVmax of the tonsils (SUVmaxTon)] had the maximum area under the curve (0.923; 95% confidence interval, 0.834–1.000), and the cut-off value was 0.264 according to the maximum value of Youden’s index. The sensitivity, specificity, positive predictive value, negative predictive value, and accuracy were 93.6%, 94.7%, 97.8%, 85.7%, and 93.9%, respectively. Conclusions: (SDL/LDL)*(SUVmaxBio/SUVmaxTon) has good sensitivity, specificity, positive predictive and negative predictive values, and accuracy, and can be used as a good quantitative index for the diagnosis of non-destructive PTLD.