Composite segment has been widely used in deep shield tunnel.However,the present design for composite segment based on the theory of reinforced concrete cannot appropriately reflect the excellent properties of composite segment.Therefore,the rational design method of composite segment is required.In this paper,the proposed mechanical model is used to design the most complicated composite segment called as closed composite segment for underground discharge channel based on the limit state design method.As a result,the reduction of segment thickness is obtained using Closed-composite segment and this indicates that composite segment is suitable for decreasing muck,construction period and the outside diameter of the shield machine.In general,the costs of construction,risk,and maintenance will be decreased