A method for forming a multi-gate semiconductor device includes providing a substrate including at least a fin structure and a dummy gate structure over the fin structure and the substrate, disposing a conductive spacer over sidewalls of the dummy gate structure, portions of the fin structure are exposed from the dummy gate structure and the conductive spacer, forming a source/drain region in the portions of the fin structures exposed from the dummy gate structure and the conductive spacer, disposing a dielectric structure over the substrate, removing the dummy gate structure to form a gate trench in the dielectric structure, the conductive spacer is exposed from sidewalls of the gate trench, disposing at least a gate dielectric layer over a bottom of the gate trench, and disposing a gate conductive structure in the gate trench, sidewalls of the gate conductive structure are in contact with the conductive spacer.