Despite great improvement in the treatment outcome of APL, treatment failure still sometimes occurs due to the toxicity of arsenic trioxide (ATO). Damage to the heart and liver often occurs even when the dose is lower than the therapeutic dose. Based on the results of cell experiments in vitro in this study, we investigated the synergistic activity of carnosic acid (CA) combined with ATO in the SCID mouse model of human promyelocytic leukaemia in vivo. A NB4/SCID mouse model was established in this study. The NB4/SCID mice were randomly divided into three treatment groups (CA alone, ATO alone and CA combined with ATO) and a control group based on factorial design. The evaluation indicators of the curative effect of the drugs included expressions of cleaved caspase-3, PTEN, p27 gene mRNA and proteins by immunohistochemistry, flow cytometry and Western blot analysis. The survival time was compared between the four groups. The results indicated that verification of the NB4/SCID mouse model was confirmed by histopathological examination. Compared with mice treated by CA or ATO alone, the mice in the combination of CA and ATO group had a higher rate of apoptosis, which was linked with expressions of cleaved caspase-3, PTEN, p27 gene mRNA and proteins. Also, the mice with the longest survival time were those treated with the combination of CA and ATO. In conclusion, the results of the present study indicated that CA and ATO in combination have strong synergistic antileukaemic effects on cell activity.