Most of the researches on smoking and human health has focused on the harmful effect of smoking on health. However, some epidemiological studies have shown that smoking has a protective effect on the occurrence of some diseases, such as Parkinson's disease. This phenomenon has attracted the attention of some researchers, and many systematic reviews and meta-analyses have been carried out. However, high quality systematic reviews can provide decision basis for clinical decision makers, while low quality systematic reviews may mislead decision makers. At present, there is no research to analyses the results and quality of systematic reviews on the protective effect of smoking, as well as the protective effect of smoking on which diseases, which is still unclear.