Objective: To investigate the effects of decitabine (DEC) combined with all-trans retinoic acid (ATRA) on the number of immune cells, efficacy and adverse reactions in the treatment of myeloid neoplasms patients., Methods: Eighty-four patients with myeloid tumors, including AML, MDS-EB-1 or MDS-EB-2 treated by the regimen containing decitabine in our hospital from January 2009 to October 2019 were enrolled and retrospectively analyzed, among the patients, 21 patients treated with DEC alone, 24 patients treated with DEC combined with ATRA (DEC/ATRA) and 39 patients treated with DEC combined with G-CSF priming regimen (DEC/priming). The changes of peripheral blood immune cell levels before and after treatment of the patients between the three groups were compared, and the differences in clinical efficacy and adverse reactions of the patients between the three groups were also compared., Results: There was no statistical differences in the number of immune cells among the patients in the three groups before treatment (P>0.05). NK cell levels decreased significantly in the patients in DEC and DEC/ATRA group after treatment (P<0.05); After treatment, the levels of CD8+ and CD3+T cells in the patients treated by DEC /priming regimen significantly increased (P<0.05), while the levels of CD3-HLA-DR+ B cells significantly decreased (P<0.05). The overall response rate (ORR) of the patients in DEC/ATRA group (75%) and DEC/priming group (74.36%) was significantly higher than 42.86% in DEC monotherapy group, and the differences showed statistically significant (P<0.05), while the ORR between the patients in DEC/ATRA and DEC/priming group showed no statistic differences (P>0.05). There were no statistical differences in overall survival (OS) and incidence of bleeding between the patients in the three groups (P>0.05). The incidences of grade 3 to 4 bone marrow suppression and the infection rate of the patients in DEC monotherapy and DEC/ATRA group were significantly lower than that in DEC/priming regimen group after treatment (all P<0.05), however, there was no statistical difference between DEC monotherapy and the DEC/ATRA group., Conclusion: The efficacy of DEC/ATRA on myeloid neoplasms is comparable to that of DEC/priming regimen, and the anti-myeloid tumor effect of DEC/ATRA regimen may be related to the regulation of NK cells and T cells.