During the course of multi-cavity injection molding, it is very important for the melt plastic to flow balanceablely in the mould cavities. The feed system is balanced if the melt plastic can fill the separate mould cavity at the same time. The balanced feed system ensures not only good quality of plastic parts but also the consistency of quality of the plastic parts in the separate mould cavity. The balance of feed system in the multi-cavity mould depends on the layout of the cavities and runners. Taking a set of four-cavity injection mould for an example in this paper, the cavity layout is optimizedly designed and the optimized scheme of feed system is determined based on CAE simulation analysis on the two schemes of feed system using Moldflow software. Then the detailed structure of injection mould is designed. The cavity layout is reasonable and the mould structure is compact. The practical production validates that the molding quality of the plastic parts is well enough to satisfy the requirements of the customers.