A method for forming a multi-gate semiconductor device includes forming a fin structure including alternating stacked first semiconductor layers and second semiconductor layers over a substrate, forming a dummy gate structure across the fin structure, forming a first spacer alongside the dummy gate structure, removing a first portion of the first spacer to expose the dummy gate structure, forming a second spacer between a second portion of first spacer and the dummy gate structure after removing the first portion of the first spacer, removing the dummy gate structure to expose a sidewall of the second spacer, removing the first semiconductor layers of the fin structure to form a plurality of nanostructures from the second semiconductor layers of the fin structure, and forming a gate conductive structure to wrap around the plurality of nanostructures. The gate conductive structure is in contact with the sidewall of the second spacer.