Objective To explore the effect of incentive breathing training combined with oral decontamination on postoperative pneumonia (POP) in elderly patients with gastric cancer. Methods Elderly patients undergoing radical gastrectomy for gastric cancer at the Second Affiliated Hospital of Fujian University of Traditional Chinese Medicine from January 2019 to June 2020 were prospectively collected and randomly divided into experimental group and control group. Control group received routine care during peri-operative period, experimental group used incentive spirometer training combined with chlorhexidine oral decontamination on the basis of routing care. Incidence of POP, oral and pharyngeal bacterial colonization and other indicators in two groups of patients were analyzed. Results A total of 96 elderly patients with gastric cancer were included, 48 cases in experimental group and 48 in control group. 13 patients (13.54%) had POP. Incidence of POP in patients in experimental group was 6.25%, which was lower than 20.83% in control group, with a significant difference (P<0.05). After intervention, isolation rate of oral-pharyngeal colonized bacteria in experimental group was lower than that in control group (6.25% vs 20.83%, P<0.05); after intervention, 6-minute walking test and compliance to respiratory training in patients in experimental group were both higher than control group (both P<0.05); duration of intra-operative mechanical ventilation, postoperative hospital stay, cost of antimicrobial agents and total hospitalization expenses in experimental group were all lower than those in control group (all P<0.05). Conclusion Peri-operative incentive breathing training and oral decontamination in elderly patients with gastric cancer can help to improve exercise tolerance, reduce respiratory tract microbial colonization, decrease incidence of POP, and promote recovery of patients.