A folding treadmill offering buffering effect during folding and unfolding comprises a base that has a circular tread belt as well as a power cylinder. The power cylinder includes a pressurization chamber and an oil-air cylinder chamber, wherein the oil-air cylinder chamber is loaded with pressured air and oil of a predetermined amount. Due to velocity difference between air and oil, a phased-changeable deceleration resistance can be provided during folding and unfolding the treadmill. Further, the characteristics of the power cylinder is related to buffer and balance so as to achieve the buffering effect in both lifting and lowering of the base. Thereupon, a safe altitude of the base can be defined and a designed structure contributing to users' safety can be accomplished.