A thin key structure includes a keycap, a key body, an elastic piece and a base. The keycap has a first side wall. The key body has a second side wall forming a hollow chamber in which the keycap can be put to connect the elastic piece. The inner of the second side wall is provided with guiding portions for guiding the keycap to move steadily in the key body. With the keycap being put in the key body, the total height of the key structure can be reduced, and the vertical moving distance of the keycap with respect to the key body to press the elastic piece can be shortened. Thus, the thin key structure can be made more compact.