Embodiments of a three-dimensional (3D) memory device with a corrosion-resistant composite spacer and method for forming the same are disclosed. In an example, a method for forming a 3D memory device is disclosed. A dielectric stack including a plurality of dielectric/sacrificial layer pairs is formed on a substrate. A memory string extending vertically through the dielectric stack is formed. A slit extending vertically through the dielectric stack is formed. A memory stack is formed on the substrate including a plurality of conductor/dielectric layer pairs by replacing, with a plurality of conductor layers, the sacrificial layers in the dielectric/sacrificial layer pairs through the slit. A composite spacer is formed along a sidewall of the slit. The composite spacer includes a first silicon oxide film, a second silicon oxide film, and a dielectric film formed laterally between the first silicon oxide film and the second silicon oxide film. A slit contact extending vertically in the slit is formed.