Recombinant Anti-CD20 x Anti-CD22 Tandem scFv-SAR immunotoxin (Bispecific Immunotoxin) is designed to be expressed as the α-Sarcin (SAR) fused to a tandem scFv recognizing CD20 and CD22. It contains a His-tag at the C/N terminus for affinity purification. This bispecific immunotoxin can deliver SAR to antigen-specific cells. Then SAR is a ribonuclease which can inhibit protein synthesis and kill the target cells. The bispecific immunotoxin is designed for the research of Lymphoma; Leukemia; Antoimmune disease; Immune dysfunction disease therapy.