A method for manufacturing a polysilicon SOI substrate including a cavity. The method includes: providing a silicon substrate including a sacrificial layer thereon; producing a first polysilicon layer on the sacrificial layer; depositing a structuring layer on the first polysilicon layer; introducing trenches through the structuring layer, the first polysilicon layer, and the sacrificial layer up to the silicon substrate; producing a cavity in the silicon substrate by etching, an etching medium being conducted thereto through the trenches; producing a second polysilicon layer on the first polysilicon layer, the trenches being thereby closed. A micromechanical device is also described.