According to embodiments, a sheet post-processing apparatus includes a processing tray, a vertical alignment section, a horizontal alignment section and a controller. The controller is configured to control the horizontal alignment section to shift the sheet to a first alignment position a predetermined distance from the center of the processing tray in the sheet width direction, control the horizontal alignment section to align the sheet in the sheet width direction at the first alignment position, and control the vertical alignment section to align the sheet in the conveying direction at the first alignment position, control the horizontal alignment section to realign the sheet at the first alignment position.