ObjectiveTo compare the effect of diaphragmatic and abdominal breathing training on the morphology and function of deep trunk muscles in patients with chronic nonspecific low back pain. MethodsFrom March to May, 2025, a total of 37 patients with chronic nonspecific low back pain recruited at Xi 'an Physical Education University, were randomly divided into control group (n = 12), abdominal breathing group (n = 12) and diaphragmatic breathing group (n = 13). All groups received transcutaneous electrical nerve stimulation and core stability training. On this basis, the abdominal breathing group was added with abdominal breathing training, and the diaphragmatic breathing group was added with diaphragmatic breathing training, for six weeks. The degree of pain was evaluated with Visual Analogue Scale (VAS) before and after intervention, while the activation levels of multifidus and transversus abdominis were evaluated with the root mean square value (RMS) of surface electromyography, and the thickness of the multifidus muscle, transverse abdominis muscle, diaphragm and diaphragm mobility were evaluated with musculoskeletal ultrasound. ResultsAfter intervention, all the indicators improved in all the groups (P < 0.001), except for the diaphragm thickness and mobility in the control group. All the effects (F > 4.634, P < 0.016) of VAS score were significant, and were better in the diaphragmatic/abdominal breathing groups than in the control group (P < 0.01). The intra-group effects (F > 64.869, P < 0.001) and interaction effects (F > 12.379, P < 0.001) of muscle morphology and function indexes were significant, and the RMS of multifidus and transversus abdominis, left multifidus thickness, bilateral transversus abdominis thickness, diaphragm thickness, and diaphragm excursion was better in the diaphragmatic breathing group than in the control group and the abdominal breathing group (P < 0.001), while bilateral transversus abdominis RMS, diaphragm excursion, and right multifidus thickness were better in the abdominal breathing group than in the control group (P < 0.037). ConclusionThe combination of diaphragmatic/abdominal breathing exercises can further improve the morphology and function of the deep muscles of trunk in patients with chronic nonspecific low back pain, and diaphragmatic breathing may be superior to abdominal breathing.