A method for manufacturing a buried-strap includes: forming a trench capacitor structure in a semiconductor substrate, wherein the trench capacitor structure has a doped polysilicon layer and an isolation collar covered by the doped polysilicon layer, and a top surface of the doped polysilicon layer is lower than a top surface of the semiconductor substrate such that a first recess is formed; sequentially forming a first resist layer, a second resist layer and a third resist layer over the semiconductor substrate; sequentially patterning the third resist layer, the second resist layer and the first resist layer, forming a patterned tri-layer resist layer over the semiconductor substrate; partially removing a portion of the doped polysilicon layer exposed by the patterned tri-layer resist layer to form a second recess; removing the patterned tri-layer resist layer; and forming an insulating layer in the second recess and a portion of the first recess.