Abstract: A vertical axis windmill with a deceleration control system includes: a stationary shaft, a rotary shaft rotatably sleeved onto the stationary shaft, plural blades fixed to the rotary shaft, and a generator disposed between the stationary shaft and the rotary shaft. The deceleration control system includes a damping plate, a magnetic-force generating unit, a rectifier, and a control unit. The magnetic-force generating unit generates an eddy current to produce a damping effect. The rectifier converts AC voltage outputted from the generator into DC voltage. The control unit is coupled between the rectifier and the magnetic-force generating unit and sets a conduction level. When the DC voltage is lower than the conduction level, the rectifier and the magnetic-force generating unit are electrically disconnected from each other, and when the DC voltage is greater than the conduction level, the rectifier and the magnetic-force generating unit are electrically connected to each other.