【Objective】 To investigate the clinical characteristics and diagnosis and treatment of passenger lymphocyte syndrome (PLS) in patients following allogeneic hematopoietic stem cell transplantation (allo-HSCT). 【Methods】 A total of 489 patients who underwent allo-HSCT in Suzhou Hongci Hematology Hospital were retrospectively enrolled. The clinical process, diagnosis and treatment measures and prognosis of four patients complicated with PLS after transplantation were analyzed. 【Results】 Among the 489 patients, 4 were diagnosed with PLS. The blood types of donor/recipient ABO were all secondary incompatible (The blood type of donors were O and the recipients were A or B). The overall incidence of PLS in allo-HSCT was 0.82%(4/489)and 2.2%(4/179)in transplants with donor/recipient secondary incompatible ABO-blood types. PLS occured in 6-13 days after donor stem cell infusion. Clinical manifestations were dizziness and fatigue, low back pain, jaundice, deepening urine, rapid decrease in hemoglobin on laboratory tests, elevated indirect bilirubin and lactate dehydrogenase, positive urobilinogen, positive direct anti-human globulin test (DAT), and anti-A or anti-B antibodies against recipient red blood cells were detected in plasma. After the treatment of O-type washed red blood cells, methylprednisolone, gamma globulin, rituximab and other treatments, the hemolysis was improved. All patients achieved engraftment of neutrophil and platelet. Red blood cell transfusion was halted in 3 weeks. 【Conclusion】 PLS is a rare complication of allo-HSCT, which mainly occurs in allo-HSCT patients with secondary incompatibility of ABO blood group of donor/recipient. The clinical prognosis is good after properly treatment.