The etiology of hemorrhagic cystitis after hematopoietic stem cell transplantation includes those related to the chemotherapeutic agent cyclophosphamide (requiring treatment such as alkalinization of urine, diuresis, and so on) and those related to viral infection and immune response (requiring treatment such as antiviral, hydration, and so on), which need to be analyzed specifically. 1. Related to the chemotherapeutic drug cyclophosphamide: a common adverse reaction to cyclophosphamide is hemorrhagic cystitis, which occurs with subsequent alkalinization of urine, diuresis, and symptomatic treatment such as bladder irrigation. 2. Related to viral infection and immune response: hemorrhagic cystitis caused by this etiology requires antiviral treatment, commonly used drugs include ribavirin, etc., which can be injected systemically or intravesically, and glucocorticosteroids can be applied to control the infection and immune response if necessary, and other treatments include diuresis, alkalinization of urine, and bladder irrigation. The treatment of hemorrhagic cystitis after hematopoietic stem cell transplantation should be carried out correctly under the guidance of doctors.