Recombinant Anti-CD20 x Anti-CD22 Tandem Fab-MEL immunotoxin (Bispecific Immunotoxin) is designed to be expressed as the Melittin (MEL) fused to a tandem Fab recognizing CD20 and CD22. It contains three chains and a His-tag at the C/N terminus for affinity purification. The two Fd regions are connected by a flexible linker. This bispecific immunotoxin can deliver MEL to antigen-specific cells. Then MEL is a cytolytic peptide and can kill the target cells. The bispecific immunotoxin is designed for the research of Lymphoma; Leukemia; Antoimmune disease; Immune dysfunction disease therapy.