According to one embodiment, a sheet post-processing apparatus has a sheet discharge tray, a drive motor, a power transmission breaker, a first power transmitter, a second power transmitter, and a load reducer. The sheet discharge tray stacks sheets. The drive motor raises and lowers the sheet discharge tray. The power transmission breaker can cut off power transmission from the drive motor to the sheet discharge tray when an upward external force is applied to the sheet discharge tray. The load reducer provides a reverse load to the sheet discharge tray in a reverse direction to the load generated on the sheet discharge tray due to stacking of the sheets. The load reducer is connected to the first power transmitter.