The design of FMS is different from that of ordinary mechanical manufacturing system，the design of which can be carried out by some mathe-matical formulas. The FMS projects can not be optimized by physical test and they can be tested only by simulation. In this paper，ACD diagram is used to analyze and to model FMS. Through an example，the FMS model set is simulated by language GPSS. Some problems in the design of FMS are presented and the foundation to solve them is found.