A boy aged 5 years and 2 months with a history of acute myeloid leukemia of more than 10 months was admitted. Twenty days after admission, he underwent an HLA 8/10-matched unrelated umbilical cord blood stem cell transplantation. On day 6 post-transplant, high fever and a generalized erythematous papular rash occurred, with elevations of liver enzymes and interleukin-6; pre-engraftment syndrome was diagnosed and symptoms improved after symptomatic treatment. On day 22 post-transplant, refractory hyponatremia occurred. After drug effects, infection, central nervous system lesions, other organ dysfunction, and renal salt wasting were excluded, syndrome of inappropriate antidiuretic hormone secretion (SIADH) was diagnosed. Fluid restriction, 3% sodium chloride infusion, and oral salt capsules were administered, and SIADH resolved completely as the pre-engraftment syndrome resolved. One pediatric case of SIADH after hematopoietic stem cell transplantation is reported, and published cases are summarized to inform early diagnosis and treatment.