The mechanism of acute lung injury (ALI) following limb ischemia-reperfusion (LIR) is not yet clear. We speculate that the unbalanced expression of angiotensin-converting enzymes (ACE and ACE2) and angiotensins [Ang II and Ang-(1-7)] in the renin-angiotensin system (RAS) is a major cause of ALI. To prove this hypothesis, pathological changes, lung edema, and permeability of wild-type mice at different time points within 12 h of reperfusion after 2 h of hind-limb ischemia were first detected by morphological method, measurements of wet-to-dry weight ratio, and bronchoalveolar lavage fluid. Meanwhile, the changes of lung ACE/ACE2 mRNA and protein expression were surveyed by the methods of real-time reverse transcription-polymerase chain reaction, Western blotting, and immunohistochemistry. Angiotensin II/Ang-(1-7) levels in the blood serum and lung tissue were measured by enzyme-linked immunosorbent assay. Then the effects of ACE2 gene insertion and deletion on the previously mentioned parameters were investigated in the mice being exposed to hind-limb 2-h ischemia and 4-h reperfusion. The results revealed that lung injuries in the wild-type mice were gradually aggravated, and the expression of ACE in lung tissue was progressively increased, whereas that of ACE2 decreased within 12 h after LIR. Unexpectedly, both Ang II and Ang-(1-7) in the lung tissue were obviously increased after LIR, showing Ang-(1-7) higher than Ang II in the early stage of reperfusion but lower than Ang II at the late stage of reperfusion. Unlike local Ang II/Ang-(1-7) changes, circulating Ang-(1-7) became greatly descending, and Ang II was markedly ascending from the start of reperfusion, corresponding to local ACE/ACE2 unbalanced expression. ACE2 transgenosis improved the imbalance of ACE/ACE2 and Ang II/Ang-(1-7) expression and alleviated lung injuries, whereas ACE2 knockout further aggravated the imbalance of ACE/ACE2 and Ang II/Ang-(1-7) expression and made lung injuries more serious in the post-LIR mice. The results indicate that the dysregulation of local and circulating RAS with increased expression of ACE/Ang II and decreased expression of ACE2/Ang-(1-7) contribute to ALI caused by LIR in mice. Maintaining RAS homeostasis through upregulating ACE2 expression may lessen lung injury, which provides a new idea for the treatment of posttraumatic ALI.