The present invention discloses a blind slat positioning device and a window blind thereof. The positioning device can fix the slat at any height at which the slat is pulled down. The positioning device includes a turning seat which is provided at least with an overbend element for a pull cord to wrap around. The overbend element includes at least a ridge, resulting an effect that the slat stops at the height at which the slat is pulled down, with the aid of gravity.