Agent technology is regared as an effective approach for manufacturing system modeling, and it is widely used in manufacturing job shop scheduling. In the job shop scheduling based on contract net protocol, repeated negotiations are required and the problem of concurrent-tendering operation emerges. To increase working efficiency of contract net protocol, the protocol is simplified and an improved contract net protocol based on cycle period is proposed. The protocol deals with the tender orderly, and introduces the threshold value into the negotiations, which limits the bidding price. Finally, the experiments based on multi-agent job shop AGV scheduling validate the effect of the improved contract net protocol based on cycle period.