The invention relates to an internal combustion engine, in particular to a crankshaft-free single-cylinder three-piston internal combustion engine. In the technical field of internal combustion engines, a traditional four-stroke internal combustion engine occupies a dominant position at present, and then the defects of large size, high energy consumption, high noise and large weight of the traditional four-stroke internal combustion engine cannot be thoroughly overcome. The crankshaft-free single-cylinder three-piston internal combustion engine can be a new method for replacing the traditionalfour-stroke internal combustion engine. The crankshaft-free single-cylinder three-piston internal combustion engine specifically comprises a machine body, a machine cover, a spoke and pistons, wherein the machine body is similar to a circle and is composed of a plurality of curvature radiuses, the machine cover is of a circular structure, the spoke is of a circular structure, and the pistons aresimilar to rectangular cross sections. The crankshaft-free single-cylinder three-piston internal combustion engine has the following beneficial effects that 1, the structure is simple and easy to develop; 2, the operation is stable, and noise is low; 3, the volume is small, and the weight is low; and 4, the energy-saving potential is large, specifically, the main technical advantage of the projectlies in that compared with a traditional internal combustion engine, the crankshaft-free single-cylinder three-piston internal combustion engine has a much larger S/D ratio, and therefore a great energy-saving potential is created for the project.