For enterprises whose bulk raw materials are supplied by multiple suppliers, the train departure interval of suppliers may affect their inventory level. This paper focuses on the linkage departure interval of entire direct-trains between multi-suppliers under the target of minimum average inventory of the customer. Due to the fact that the arrival interval will be influenced by the supply volume from different suppliers, freight volume of originating direct train, and the delivery time or path differences in transit, it respectively analyzes the optimization methods of linkage departure interval under four combinations of freight volume for multi-suppliers and the annual supply volume of each supplier. Finally, the corresponding numerical example is given, and the results show that the average inventory level of customer is reduced by 24.2% compared to the random departure interval.