The invention discloses a crankshaft-free engine mechanical structure which comprises a crank link mechanism, a cylinder, a cylinder cover, a conical gear case, a cylindrical gear case, a main shaft and other main parts. The main shaft is connected with the cylinder, the conical gear case and the cylindrical gear case, and the crank link mechanism is connected with the conical gear case. The reciprocating movement of a piston is changed into the rotation of cranks through crank links, the stress is improved by the links through staggered structural design, each crank is connected with a smallbevel gear, the cylinder rotates along with the main shaft, and the rotating speed difference between the main shaft and the conical gear case is balanced through the cylindrical gear case; a crankshaft is replaced with a transmission gear, so the problems and troubles caused by using the crankshaft are solved, and an engine is more compact in structure and higher in work efficiency.