An etching method and a fabrication method of semiconductor structures are provided. The etching method includes forming trenches in a to-be-etched structure, and forming a dielectric layer in the trenches. The etching method further includes etching the dielectric layer in the trenches by an etching process, and controlling at least an etching temperature of the etching process while a polymer is formed on side surface of the to-be-etched structure. During the etching process of the dielectric layer, the polymer undergoes a deposition stage and a removal stage. The deposition stage has a deposition rate of the polymer greater than an etch rate of the polymer, and the removal stage has the deposition rate of the polymer less than the etch rate of the polymer.