A spacer includes: projections each of which has a frame; multiple standing portions each of which has one end connected to the frame and that stand on the frame; and a standing-end connector that connects other ends of the standing portions, the frames being disposed close to each other such that the projections are connected; and flexible connectors that connect the frames. The projections have a three-dimensional shape. The flexible connectors are elastically deformable. At least either the projections or the flexible connectors have different shapes depending on a part of the spacer.

A seat structure with an adjustable sense of sitting comprises a cover and a base plate, wherein an adjusting cavity is defined by the base plate and the cover; at least two soft inner layers different in hardness and breathability are arranged in the adjusting cavity, and the soft inner layers are at least two of a sponge cushion, a 3D mesh cushion and an air fiber cushion; and a cover body is provided with a zipper structure, which can be slidably unzipped to form an opening for adjusting the soft inner layers. The soft inner layers can be taken out via the opening, and users can place multiple soft inner layers in the adjusting cavity in different sequences as required to adjust the hardness and breathability of the seat structure to obtain suitable hardness and breathability so as to improve sitting comfort.