Objective:To preliminarily investigate the safety and efficacy of programmed death-1(PD-1)inhibitor combined with cord blood-derived natural killer cells(NK cells)in the treatment of advanced malignant solid tumors in an exploratory clinical trial.Methods:Three patients with advanced solid tumors treated at the Second Affiliated Hospital of Xi'an Medical University from December 2019 to December 2021 were enrolled.According to tumor type and CSCO guidelines,patients received multiple treatment cycles(21 days per cycle)consisting of standard chemotherapy,targeted therapy,or bevacizumab combined with PD-1 inhibitor.Umbilical cord blood-derived NK cells(8×107 cells per infusion)were infused at appropriate intervals during the treatment course.Target lesion size,tumor markers,levels of 12 peripheral blood cytokines,and lymphocyte subsets were assessed in each treatment cycle.Adverse events were also monitored throughout the treatment.Results:Following the treatment with PD-1 inhibitor combined with cord blood NK cells,2 patients achieved stable disease(SD,per RECIST 1.1 criteria),with durations of 118 days and 92 days,respectively.After NK cell infusion,patient#1 exhibited a marked decrease in the tumor marker CA199 to normal range and sustained for three follow-up periods;patient#2 showed significant reductions in tumor markers CA199,CA242,and CA724.Conclusion:The combination of NK cells with chemotherapy and PD-1 inhibitor demonstrates potential therapeutic efficacy for solid tumors.No severe immune-related adverse reactions were observed in the three patients enrolled in this study.