A portable chair has a mechanism for self folding up and cushioning a seat. The mechanism includes a tension spring assembled between a seat frame and a rear leg for collapsing the seat, and a cushioning pad affixed on the rear leg. The tension spring has first and second hook elements disposed on different orientations and engaged with holes. The hook elements are easy to install, and reduce the risk of disengagement after assembly. The length of the spring is substantially equal to the length of the space in which it is assembled, so that it is not excessively stretched during assembly, thereby preventing spring fatigue. The cushioning pad is integrally formed and includes retainer and insert portions through which it is affixed with the rear leg, and a cushion portion that can absorb noise and vibration when the seat is collapsed.