Abstract: Researches, especially population studies and clinical studies, need to be repeated. However, repeated studies on the same question may produce conflict results. Therefore, a systematic review, a timely comprehensive analysis of individual studies on the same question, is the basis of evidence-based decision making. Meta-analysis of systematic review, which is regarded as the method for producing the evidence of top level, has been widely used in more and more fields. However, there are some methodological limitations for meta-analysis. It is controversial whether meta-analysis could be applicable to all type of researches. In view of this fact, we introduced the procedure of systematic review, the basic principle and the statistical formula of meta-analysis, and further explored the sources of error in meta-analysis and explained the difference between the random effect model and the fixed effect model in order to understand the methodological limitations of meta-analysis and perform it deliberately. At the same time, we recommended that clinicians should carefully interpret the pooled results of a meta-analysis, especially of the one using the random effect model, when they are going to make a clinical decision.