Xiaolangdi sand drainage tunnel undertakes the tasks of sand drainage and partial flood discharge of the reservoir, with an applied water head of 122m. In order to prevent the lining cracking under the action of high pressure water head, the original design adopts the bonded prestressed concrete lining scheme. After field test research and calculation analysis, it is decided to change to the unbonded prestressed structural lining scheme. The design and research of the new scheme includes the structural layout of unbonded prestressed concrete lining, the stress characteristic analysis of prestressed concrete structure, the parameter demonstration and selection of prestressed components and equipment.