Clinical efficacy and adverse reactions of pemetrexed combined with stereotactic gamma-ray radiotherapy in the treatment of patients with lung adenocarcinoma brain metastasis in The First People's Hospital of Yunnan Province were evaluated. A total of 67 patients with lung adenocarcinoma brain metastasis in experimental group were treated with simple pemetrexed chemotherapy, and then with radiotherapy, followed by pemetrexed chemotherapy. Their treatment results were compared with those of 53 patients treated with simple gamma knife in control group. The results were analyzed by comparing the clinical efficacy, side reactions, serum level changes, and survival between the two groups. Among 67 patients in the experimental group, there were 16 cases of complete response (CR), 39 cases of partial response (PR), 7 cases of stable disease (SD) and 5 cases of progressive disease (PD), with an effective rate of 82.09% (55/67) and a tumor local control rate of 92.54% (62/67). Among 53 patients in the control group, there were 13 cases of CR, 20 cases of PR, 9 cases of SD and 11 cases of PD, with an effective rate of 62.26% (33/53) and a tumor local control rate of 79.25% (42/53). There were statistically significant differences in the effective rate and local control rate between the two groups (P<0.05). The 6-, 12- and 24-month survival rates in experimental group were higher than those in control group (P<0.05). The main adverse reactions after pemetrexed combined with radiotherapy were lower than those after simple radiotherapy (P<0.05). The expression levels of the tumor markers carcinoembryonic antigen (CEA) and cytokeratin fragment antigen 21-1 (CYFRA21-1) in the two groups of patients after treatment were lower than those before treatment (P<0.05). After treatment, the expression levels of serum CEA and CYFRA21-1 in the experimental group were significantly lower than those in the control group (P<0.05). Pemetrexed combined with radiotherapy in the treatment of lung adenocarcinoma brain metastasis is more effective than simple radiotherapy, with lighter adverse reactions, worthy of clinical application and promotion.