Abstract: Value stream mapping is used to analyze traditional machinery factory. Lots of problems like confusion of logistics and imbalance of the production line are found. After that, lean production is adopted to deal with these problems. However, only a period of time in factory can be described by value stream mapping. Flexsim is a key point in dealing with shortcomings of value stream mapping. After analysis of lean production, scenario of improvement can be verified by Flexsim. A final scenario would be established by a statistic report of Flexsim and lean skills.