We evaluated the effects of down-regulated heme oxygenase (HO)-1 expression on the proliferation of the acute myelocytic leukemia Kasumi-1 cell line by using the HO-1 inhibitor zinc protoporphyrin IX (ZnPPIX) in combination with daunorubicin (DNR), and evaluated the mechanism. The proliferation rates of cells treated with 10 mg/mL DNR and 10 mM ZnPPIX individually or in combination for different time periods were detected using the MTT assay. The apoptotic outcomes of the blank control, ZnPPIX, DNR, and ZnPPIX groups in combination with the DNR group were detected by flow cytometry. The expression of HO-1, activating transcription factor 4, CCAAT-enhancer-binding protein homologous protein, and inositol-requiring enzyme-α mRNA and proteins were detected by fluorescent quantitative real-time polymerase chain reaction and western blotting, respectively. Combined administration inhibited the cells most potently and time-dependently, decreased the expression of HO-1, and significantly increased the expression of activating transcription factor 4, CCAAT-enhancer-binding protein homologous protein, and inositol-requiring enzyme-α expression levels. The cell apoptotic rates in the blank control, DNR, ZnPPIX, and combined administration groups were 8.32 ± 0.53, 39.16 ± 1.46, 10.46 ± 0.88, and 56.26 ± 2.24%, respectively. Inhibiting HO-1 expression can enhance the damaging effects of DNR on Kasumi-1 cells, providing experimental evidence for the improvement of therapeutic effects on acute myelocytic leukemia in clinical practice.