【Abstract】Objective To investigate the relationship of blood routine changes with the risk of bone marrow suppression in patients with lung cancer after chemotherapy. Methods Between September 2009 and September 2019, 140 lung cancer patients after chemotherapy in three hospitals were retrospectively analyzed. 68 cases appeared severe bone marrow suppression and 72 cases appeared mild bone marrow suppression. The correlation of patients’ routine blood changes such as mononuclear cells, lymphocytes and neutrophils were analyzed. The risk of bone marrow suppression was evaluated. Results There was no significant differences in age, sex, white blood cell level, hemoglobin level, albumin level,and dietary intake reduction between patients with severe bone marrow suppression and those with mild bone marrow suppression (P＞005). The significant decrease of mononuclear cell count in early stage was an independent risk factor for severe bone marrow suppression (OR=0532, 95% CI=11573809，2=0876，P=0034). After chemotherapy, the number of monocytes in blood was positively correlated with the number of neutrophils decreased, which the Pearson coefficient was 0673 (P＜005).Conclusion Lung cancer patients with early mononuclear cell count decreased after chemotherapy have a high risk of severe bone marrow suppression, which they needed clinically early intervention.