Objective To explore the effects of different intensity of inspiratory muscle training on pulmonary function, hospitalisation time and quality of life of critically ill patients. Methods A total of 42 patients were included in the intensive care ward, high dependency ward, and cardiac surgery ward of the Southern Theatre General Hospital from January 2023 to June 2023, and were randomly divided into 14 cases in the control group, 14 cases in the low-intensity inspiratory muscle training group, and 14 cases in the moderate-intensity inspiratory muscle training group; conventional treatment was used for the control group, and 20% of the maximum inspiratory pressure (MIP) was added as the starting threshold load for inspiratory muscle training for the patients of the low-intensity inspiratory muscle training group on the basis of conventional treatment. In the control group, conventional treatment was used; in the low-intensity inspiratory muscle training group, 20% of the maximum inspiratory pressure (MIP) was added to the conventional treatment as the starting threshold load for inspiratory muscle training; in the medium-intensity inspiratory muscle training group, 40% of the MIP was added to the conventional treatment as the starting threshold load for inspiratory muscle training; and the maximal inspiratory pressure, exertional lung capacity, diaphragm thickness and mobility, hospital stay and quality of life were evaluated after 6 weeks of training. Results A total of 40 patients completed the training, and the three groups showed statistically significant differences in MIP (P<0.05) and statistically significant differences in exertional lung capacity (P<0.05). There was no statistically significant difference in diaphragm thickness (P=0.566), and diaphragm mobility was lower in the control group than in the low-intensity inspiratory muscle training group (P=0.021), and there was also a difference between the low-intensity inspiratory muscle training group and the moderate-intensity inspiratory muscle training group (P=0.036); there was a difference in the length of stay in the care unit among the three groups (P=0.034), and there was no statistically different (P=0.149), and the duration of hospital stay was significantly shorter in the moderate intensity inspiratory muscle training group compared to the control group (P=0.016). Medium-intensity inspiratory muscle training can significantly improve patients' quality of life compared with the control group and low-intensity inspiratory muscle training group (P<0.05). Conclusions Both low-intensity inspiratory muscle training and moderate-intensity inspiratory muscle training can enhance the lung function of critically ill patients, improve their diaphragm mobility, and promote their pulmonary rehabilitation. Medium-intensity inspiratory muscle training was significantly better than low-intensity inspiratory muscle training in improving the lung function of patients. Moreover, moderate-intensity inspiratory muscle training may have positive significance in improving patients' quality of life and shortening their hospitalisation time.