The invention provides an internal reverse gear structure and an engine, and relates to the technical field of engine transmission devices. The internal reverse gear structure comprises a left crankcase, a shifting unit, a shifting fork and a gear shifting unit; the left crankcase is provided with a first mounting hole, a second mounting hole and a mounting groove; one end of the shifting unit issuspended, and the other end of the shifting unit penetrates through the first mounting hole to be connected with one end of the shifting fork; one end of the gear shifting unit penetrates through thesecond mounting hole to be connected with a drive shaft of a wheel, and the other end of the gear shifting unit is connected into the mounting groove; and the other end of the shifting fork is connected with the gear shifting unit, by pressing one end of the shifting unit, the shifting fork is driven by the shifting unit to drive the gear shifting unit to shift gears, and thus the engine moves forwards or backwards. The technical problem that in the prior art, a reverse gear structure is arranged outside an engine, and potential safety hazards exist for users is solved. The reverse gear structure is arranged in the engine, and the safety of the users is ensured.