A power-split transmission device for a vehicle, which connects a drive engine to a drive output, having a hydrostatic unit for continuous adjustment of the transmission ratio at a transmission unit, and two range clutches that alternately operate for respectively associated driving ranges with different transmission ratios. A control unit, during regular vehicle deceleration, implements the driving range change from a first range, with higher transmission ratios, to a second range, with lower transmission ratios, by switching from the first to the second range clutch in accordance with a synchronous point dependent deceleration control logic. In the special case of an unexpected increase of the deceleration dynamic at the beginning of an already initiated driving range change from the first to the second driving range, the control unit immediately forces the change by bypassing the synchronous point dependent deceleration control logic when other boundary conditions are fulfilled.