A mini air pump which includes a diaphragm, a bladder base provided with penetrating bladder holes and air inlet channels, a pump body having a valve seat and a pump cover, an air inlet valve, an air outlet valve, a relief valve and a preload member is revealed. The valve seat is stacked over the diaphragm. An exhaust chamber and a spring chamber are constructed by the valve seat and the pump cover. The valve seat has exhaust channels communicating with the bladder cavity. A communicating return channel is constructed by the diaphragm, the bladder base and the valve seat. The return channel is communicating with the air inlet channel but not communicating with the spring chamber. A spring of the preload member applies a preload to the relief valve. Thereby the relief valve acts more accurately and the mini air pump is more stable in use.