Objective To investigate the change law of the intracapsular pressure in vitro without outside force and the pressure of the expander upon the skin soft tissue in vivo during clinical routine expansion so as to provide some references for the safe application of the expander. Methods The rectangle expanders of 50, 80, 100, 150, 200, 250, 300, and 400 mL were used for in vitro expansion at room temperature to 400% volume of the expander capacity. The pressures before and after saline injection were recorded. Twelve patients who needed scar plastic surgery were enrolled; 17 rectangle expanders were implanted in 5 areas (cheek, trunk, forehead and temporal, limb, and head) and expanded routinely. The pressures before and after saline injection were recorded. The pressure of the expander upon the skin soft tissue was calculated and the values of the pressure at 50%, 100%, 150%, and 200% volume of 5 areas were chosen and analyzed statistically. Results The intracapsular pressure of the expanders at different volumes in vitro without outside force during routine expansion before and after saline injection was beyond 0 mm Hg (1 mm Hg=0.133 kPa) at around 100% volume, increased rapidly from 100% to 250% volume, and kept stable from 250% to 400% volume. In vivo, 16 expanders within 200% volume had the maximum pressure before saline injection, 15 had the maximum pressure after saline injection. Before saline injection, the pressure of the expander upon the skin soft tissue was lowest in the cheek, showing significant difference when compared with those of the forehead and temporal and head (P lt; 0.05); the pressure in the trunk was significantly lower than that in the head (P lt; 0.05); and there was no significant difference between the other body sites (P gt; 0.05). After saline injection, the pressure of the expander upon the skin soft tissue was lowest in the cheek, and showed an increasing trend in the trunk, the limb, the forehead and temporal, and the head; no significant difference was found between in the cheek and in the trunk, and between in the forehead and temporal and in the limb (P gt; 0.05), and significant differences were found between the other body sites (P lt; 0.05). Conclusion The pressure of expander upon skin soft tissue at early stage or middle stage is higher than that at late stage during expansion. The pressure is high in the head, and low in the cheek comparatively, and the pressures in the trunk, the forehead and temporal, and the limb are between them.