The invention discloses a grain harvesting machine with good power performance. The grain harvesting machine comprises a self-propelled machine body, a harvester, a spiral conveying device and a drum-type threshing device, wherein the harvester, the spiral conveying device and the drum-type threshing device are arranged on the self-propelled machine body; the self-propelled machine body comprises a frame, traveling wheels, an engine and a transmission component; the transmission component is in transmission connection to the power output shaft of the engine, the harvester, the spiral conveying device, the threshing device and the traveling wheels and comprises a side-hanging-type transmission box which is transversely arranged on the lateral side of the frame and is used for transmitting power to the traveling wheels; the flywheel of the engine comprises a flywheel body, at least two variable-inertia sliders and an internal balance rotary disc; the variable-inertia sliders are uniformly arranged around the rotary shaft of the flywheel body and are radially and slidably connected to the flywheel body; the internal balance rotary disc serving as a synchronizing device is coaxially and rotatably connected with the flywheel body and is in slide fit with the variable-inertia sliders through a planar slideway; the planar slideway deflects along the circumferential direction of the rotary disc, so that a certain included angle is formed between the planar slideway and the sliding direction of the corresponding slider. The grain harvesting machine with good power performance has the advantages that the engine, the harvester, the threshing device and the spiral conveying device are compactly distributed, so that the operating flexibility of the whole harvesting machine can be improved, the length of a transmission chain is reduced, the stability of a transmission structure is ensured, the grain harvesting machine is less prone to failure and damage, and maintenance cost is reduced favorably.