A fast expansion structure of a toy gun buttstock is provided. The structure includes a main body, a plurality of slide bars, a plurality of coupling portions, a plurality of fixing members, a butt piece, a plurality of rotating and retaining members, a plurality of first engaging surfaces, a plurality of second engaging surfaces, a limiting member, and a first elastic member. With the above structure, a user may pull the butt piece to cause the fixing members that are provided on the slide bars to push the rotating and retaining members for rotation so as to cause the first elastic member to push the limiting member to change the limiting member from an initial condition of contacting and pushing the first engaging surfaces to a condition of contacting and pushing the second engaging surfaces.