Complications of hematopoietic stem cell transplantation are divided into three parts: 1. Complications during pretreatment: mainly the toxic side effects of high-dose radiotherapy administered before the infusion of stem cells, including toxic damage to the heart, liver, and kidneys, gastrointestinal symptoms, mouth and lip ulcers, hemorrhagic cystitis, and neurological damage. Myelosuppression caused by radiotherapy can cause granulocytopenia, thrombocytopenia, infection and bleeding, etc.; 2. Recent complications after transplantation: First, the most common is graft-versus-host disease, which means that the donor lymphocytes infused in recognize the cells of the normal recipient and it thinks they are not its own, so it causes an attack on it, divided into acute graft-versus-host disease and chronic graft-versus-host disease. Acute graft-versus-host disease usually occurs in the early post-transplant period and is the most significant post-transplant complication and one of the most common causes of death, often accumulating to the skin, gastrointestinal tract, liver and even lungs. Second, the most common complication is infection, because whenever high doses of radiotherapy are administered, there is a period of myelosuppression at this time, which can cause patients to suffer from a variety of infections due to granulocyte deficiency, including bacterial infections, fungal infections and viral infections. Thirdly, the common post-transplantation complication is graft implantation failure or graft rejection, and implantation failure is commonly referred to as the failure of normal implantation of the seeded stem cells, i.e., the seeds that were planted did not take root and germinate. Generally, if the seeds do not implant in 28 days, it is called primary implantation failure, and a second transplantation is required at that time. Graft rejection is when the transplanted species goes in and is implanted normally, but for a period of time, the graft is rejected for various reasons, and then the primary disease will relapse, such as leukemia will relapse, and aplastic anemia will return to its original state, and graft rejection also requires re-transplantation, or timely treatment. Fourth, relapse, especially after leukemia transplantation a part of the primary disease will occur relapse, which then needs to be re-treated. Fifth, delayed hemorrhagic cystitis, most of which is related to viral infection, manifested as frequent urination, urgent urination, painful urination or blood in the eyes, and in severe cases, blood clots in the urine, even blocking the urinary tract. Sixth, post-transplant lymphoproliferative disease, mostly related to viral infection, usually manifests as persistent high fever, progressive hematocrit, and enlarged lymph nodes. This complication is a rapidly progressive complication with a high mortality rate, so regular checkups and early intervention are necessary. There are other complications such as graft dysfunction, and related microangiopathy, hepatic vein obstruction syndrome, etc. 3. Long-term complications: some complications occur some time after transplantation, or even when the patient survives for a long time, for example, because of high-dose radiotherapy, the patient is prone to second tumors. In addition, some young patients can cause damage to reproductive function, some cannot have children, and some children have growth retardation after transplantation. Also, the most critical is chronic graft-versus-host disease, which usually occurs at 100 days or even longer. This is when the patient often involves the mouth and skin, manifesting as skin rashes, mossy changes, oral ulcers, oral mucosal mossy lesions, etc. In severe cases, this can lead to pulmonary complications, such as secondary pneumonia. This can lead to serious effects on the quality of life of the patient and even endanger the patient’s life. Most of the distant complications of chronic graft-versus-host disease can be controlled with timely treatment in the early stage, so patients should carefully self-manage and self-observe for early detection and treatment.