This article proposes a new noncircular gear transmission mechanism with an involute–cycloid composite tooth profile to obtain the twice unequal amplitude transmission of noncircular gears. That is, the instantaneous transmission ratio has twice obvious fluctuations with unequal amplitude in a complete rotation cycle of gear transmission. The twice unequal amplitude transmission ratio curve was designed based on Fourier and polynomial functions, the change law of the Fourier coefficient on the instantaneous transmission ratio was analyzed, and the pressure angle and contact ratio of the involute–cycloid composite tooth profile was calculated. The involute–cycloid composite tooth profile noncircular gear was machined by wire cut electrical discharge machining technology, while its meshing experiment was performed using high-speed camera technology. The results demonstrate that the instantaneous transmission ratio curve value obtained via the high-speed camera experiment was consistent with the simulation value of virtual software. Furthermore, the involute–cycloid composite tooth profile was applied in the seedling pickup mechanism of noncircular gear planetary gear train. The possibility of the application of the involute–cycloid composite tooth profile in the seedling pickup mechanism was verified by comparing the consistency of the theoretical and simulated seedling picking trajectory.