The invention provides a crankshaft main bearing cover and an aeronautic piston engine. The crankshaft main bearing cover is in a step type, and the crankshaft main bearing cover and a lower engine body of the engine are installed in a separated manner. The crankshaft main bearing cover is connected with an upper engine body through bolts. When a piston moves in a reciprocating manner, the impact on the crankshaft is transmitted to the main bearing cover and then transmitted to the upper engine body through the bolts, the impact is borne by the upper engine body, the impact of alternating stress on the lower engine body is reduced, and the situation that the lower engine body is broken because of high-cycle fatigue is effectively prevented.