OBJECTIVE： To discuss the effective way to prevent the adverse event caused by drug overdose in doctor’s orders， and to promote the safe and rational use of drugs. METHODS： Root cause analysis （RCA） method was adopted to trace and analyze the occurrence and development of one case of adverse event caused by high-risk drug overdose in medical orders retrospectively in our hospital. RCA group was organized to collect subjective and objective data， reproduce procedure and restore the event； fish bone diagram tool was used to determine the proximal and root causes， and then optimized the related workflow and put it into action. RESULTS & CONCLUSIONS： Through RCA method， It find that the root causes of the event include the management of drug dosage unit in our hospital not normative， and staff on every position paying not enough attention to the high-risk drugs. It shows that the RCA of event can effectively help hospital finding omissions and hidden risks in business process or organization structures， and then carry out feasible countermeasures to avoid possible mistakes in daily medical work.