Immunotherapy has shown promise in treating hematological malignancies, including resistant B-ALL. One approach is CAR-NK cell therapy, which involves genetically modifying natural killer (NK) cells to target specific cancer antigens. While CAR-NK therapy offers advantages over CAR-T therapy, such as reduced immune system reactions and lower production time and cost, challenges remain regarding antitumor efficacy and the tumor microenvironment. Preclinical and early clinical studies have targeted various antigens, including CD19, with CAR-NK cells in resistant B-ALL. To further investigate the potential of anti-CD19 CAR-NK cell therapy, this study aims to evaluate its safety and determine the maximum tolerated dose (MTD) in patients who have not responded to standard treatment.