Inspiratory muscle training (IMT) has preventive effects against postoperative pulmonary complications (PPCs) after upper abdominal surgery. However, its impact on diaphragmatic function has not been evaluated. This study investigated the effect of preoperative IMT on diaphragmatic excursion (DE) and prevention of PPCs for patients with esophageal cancer. This study was an unblinded, parallel, randomized controlled trial. Patients with thoracic or abdominal esophageal cancer scheduled for esophagectomy were randomized into the incentive spirometry (IS) or IMT group. During preoperative neoadjuvant chemotherapy, IS or IMT intervention was performed. The inspiratory resistance of the IMT group was consistently set at 50% maximal inspiratory pressure. The primary outcome was the amount of change in DE evaluated with ultrasonography, and the secondary outcome was the incidence of Clavien-Dindo grade II or higher PPCs. This study recruited 42 patients. Among these patients 21 were randomized into the IS or IMT group, and 2 patients dropped out from the study. Finally, 40 patients were included in this analysis. The DE of the IMT group increased significantly after the intervention. The IMT group had significantly larger DE changes than the IS group. Of the 39 patients analyzed for postoperative outcome, 5 experienced gradeII PPCs. The IMT group had a lower incidence of PPCs than the IS group. Patients with thoracic and abdominal esophageal cancer scheduled for surgery who had preoperative IMT have increased DE, which may have an important role in prevention of PPCs.