To solve the scheduling problem in flexible assembly system(FAS),the method combining hierarchical timed-delay Petri net(HTPN) model with ant colony optimization algorithm(ACOA) is adopted.In this method,the FAS is divided into a number of subassemblies and then into many flexible assembly units.With building sub-model for each unit separately and assembly scheme being traversed,the final model is determined.The method has combined the strongpoint of ACOA and HTPN and solved the problems of assembly modeling and facilities assignments optimization of FAS.The experiments show that the method is effective and can be used to guide the assembly system design.