A method for forming a semiconductor structure includes the following steps: providing a substrate having a trench in a surface; forming an isolation layer on the surface of the substrate, the isolation layer covering a side wall and a bottom wall of the trench; pretreating the isolation layer such that an initial oxide layer is formed on a surface of the isolation layer; forming an advanced oxide layer on a surface of the initial oxide layer with an atomic layer deposition process; and forming a dielectric layer on a surface of the advanced oxide layer with a spin-on dielectrics (SOD) process such that the dielectric layer fills the trench.