The spiral case of large hydraulic unit usually adopts three different embedding types including pressure preservation，cushion layer and direct embedment. In combination with the project of Xiangjiaba Underground Power Station，a combining scheme of the embedded directly spiral case with a cushion layer is given，which can be fit in features of this hydraulic unit. The results showed that the combining scheme can be recommended，which have the advantages of more uniform distribution of stress，less tensile stress and jointly bearing load with concrete.