Objective To estimate the effects of N-acetylcystein (NAC) combined with conventional treatment on the patients with stable COPD. Methods Literatures published between January 1995 and September 2010 were searched in the databases including PubMed, CHEST, CNKI, CBM, VIP and WANGFANG for collecting the randomized control trials (RCTs) of NAC combined with the conventional treatment versus the conventional treatment on patients with stable COPD. The studies were screened according to the inclusive and exclusive criteria, the data were extracted, the quality was assessed and the meta-analysis was conducted with RevMan 5.0 software. Results A total of seven RCTs including 404 patients with stable COPD were enrolled. The meta-analysis demonstrated that, a) the short-term usage of NAC could improve PaO2 (SMD=0.05 mmHg, 95%CI –0.23 to 0.32) and PaCO2 (SMD= –0.29 mmHg, 95%CI –0.76 to 0.17) without significant differences compared with the control group; and b) the NAC could significantly improve FEV1 (SMD=1.11L, 95%CI 0.69 to 1.50) and clinical symptoms (RR=17.32, 95%CI 7.11 to 42.18), and reduce the frequency of acute exacerbation (RR=0.20, 95%CI 0.07 to 0.54) with significant differences. Conclusion The NAC used in a short-term can significantly improve arterial blood gas (ABG) and pulmonary function, and it can improve clinical symptoms and reduce the frequency of acute exacerbation. But for the possibility of moderate bias due to lower quality of the included studies and unclear implementation of RCTs, this conclusion should be cautiously applied in clinic with patients’ conditions in considered and it has to be verified with more large-scale and high-quality RCTs.