Evidence-based medicine (EBM) is a kind of clinic practice where clinicians use the best and the latest available evidence to diagnose and treat patients, and both evidence providers and users need to identify and control different kinds of biases in medical research.Directed acyclic graphsis is a tool to explore the causal relationship.The possible biases in the study can be revealed in a simple graphical language.The use of directed acyclic graphs could avoid the occurrence of bias and improve the quality of medical research and better guide clinical practice.