A seat self-lifting device for a portable chair is mounted between a seat frame and a front leg of the portable chair. The seat self-lifting device includes a rotary axle, a spiral spring and a fixing rod. The rotary axle passes through the front leg and connects with a connecting plate mounted on the seat frame. The fixing rod is perpendicularly disposed on the front leg. The spiral spring has a first end connected with the rotary axle and a second end wrapped around the fixing rod. When the seat is deployed, the rotary axle is rotated along a direction and the spiral spring is deformed to accumulate a resilient force. When the user leaves the seat, the resilient force of the spiral spring rotates the rotary axle along a reverse direction for flipping the seat to a collapsed state.