〔Abstract〕 Objective To investigate the effect of antiviral prophylaxis on liver function injury and hepatitis B virus (HBV) reactivation in elderly lung cancer patients with positive hepatitis B surface antigen (HBsAg). Methods Data of 82 elderly patients with lung cancer treated by chemotherapy in Anyang Cancer Hospital from July 2019 to June 2022 were retrospectively selected. Among them, 41 patients who did not receive antiviral preventive treatment were included in the control group, and 41 patients who received antiviral preventive treatment were included in the observation group. The changes of liver function and serum immune factors before and after chemotherapy were compared between the two groups. The incidence of liver injury and adverse reaction of chemotherapy were analyzed, and the HBV reactivation rate was compared between the two groups. Results After chemotherapy, liver function indexes of the observation group was lower than that of the control group, and serum immune factor levels was higher than that of the control group, the differences were statistically significant (P < 0.05). After chemotherapy, the degree of liver function injury in the observation group was less than that of the control group, and the HBV reactivation rate was lower than that of the control group, the differences were statistically significant (P < 0.05). The incidence of thrombocytopenia, leukopenia, bone marrow suppression and renal function injury in the observation group were lower than those of the control group, and the differences were statistically significant (P < 0.05). Conclusion Antiviral prophylactic treatment can reduce the degree of liver injury and HBV reactivation in elderly lung cancer patients with positive HBsAg after chemotherapy.