Prone position is the forefront treatment due to its simple, economic and non-invasive. In clinic, it includes horizontal prone position (HPP), Head elevated tilt 15 ° prone position (HETP) and three-step-prone position (TSP)(3).Many studies have shown that HETP can allow thoracic volume increased and make abdominal movement more coordinated in preterm children, and then it is more favorable than the HPP on improving respiratory function. So HETP has been a routine position in NICU instead of HPP (4, 5). HETP, however, always make the babies slide to the foot of the bed resulting in airway obstruction. Therefore, scholars have proposed TSP which should prevent this phenomenon (6). In the study, the effectiveness of TSP on improvement of AOP will be evaluated trying to find a more suitable position for preterm infants.