A mini air pump comprising: a diaphragm which includes a plurality of bladders on one side of the diaphragm and each of the bladders having a bladder cavity with an opening toward the other side of the diaphragm; a bladder base which includes a plurality of air inlet channels and a plurality of bladder holes penetrating the bladder base in thickness direction while the diaphragm is attached to the bladder base and the bladders are passed through the bladder holes correspondingly; a pump body which includes a valve seat provided with a plurality of exhaust channels and stacked over the diaphragm, a pump cover stacked on the valve seat, an exhaust chamber and a spring chamber separated from each other and constructed by the valve seat and the pump cover, and a return channel constructed by the valve seat, the diaphragm, and the bladder base and communicating the valve seat, the diaphragm and the bladder base with one another; the exhaust channels communicating with the bladder cavity while return channel communicating with the air inlet channels but not communicating with the spring chamber; an air inlet valve disposed in flowing direction of air in the air inlet channel and used for communicating the bladder cavity with the air inlet channels in one-way manner; an air outlet valve arranged in flowing direction of air in the exhaust channel and used for communicating the exhaust chamber with the bladder cavity in one-way manner; a relief valve mounted in the exhaust chamber for selectively communicating the return channel with the exhaust chamber; and a preload member mounted in the spring chamber and having at least one spring which applies a preload to the relief valve; wherein the spring is relatively stably positioned in the spring chamber to ensure that the abutting force applied to the relief valve by the spring remains the same and the relief valve can actuate more accurately; wherein the diaphragm is a flexible member being preferably made of a rubber material; and wherein the pump body is made of an easy-moldable plastic; characterized in that the exhaust chamber and the spring chamber are separated from each other; a valve cap is disposed over the pump cover and a buffer chamber is constructed by the pump cover and the valve cap; wherein the buffer chamber is communicating with the exhaust chamber; wherein a one-way valve is arranged at an air vent of the valve cap or the pump cover; wherein the one-way valve is open when air in the exhaust chamber is delivered to the outside, whereas, while the air in the exhaust chamber is not delivered to the outside, the one-way valve is closed. a diaphragm which includes a plurality of bladders on one side of the diaphragm and each of the bladders having a bladder cavity with an opening toward the other side of the diaphragm; a bladder base which includes a plurality of air inlet channels and a plurality of bladder holes penetrating the bladder base in thickness direction while the diaphragm is attached to the bladder base and the bladders are passed through the bladder holes correspondingly; a pump body which includes a valve seat provided with a plurality of exhaust channels and stacked over the diaphragm, a pump cover stacked on the valve seat, an exhaust chamber and a spring chamber separated from each other and constructed by the valve seat and the pump cover, and a return channel constructed by the valve seat, the diaphragm, and the bladder base and communicating the valve seat, the diaphragm and the bladder base with one another; the exhaust channels communicating with the bladder cavity while return channel communicating with the air inlet channels but not communicating with the spring chamber; an air inlet valve disposed in flowing direction of air in the air inlet channel and used for communicating the bladder cavity with the air inlet channels in one-way manner; an air outlet valve arranged in flowing direction of air in the exhaust channel and used for communicating the exhaust chamber with the bladder cavity in one-way manner; a relief valve mounted in the exhaust chamber for selectively communicating the return channel with the exhaust chamber; and a preload member mounted in the spring chamber and having at least one spring which applies a preload to the relief valve; wherein the spring is relatively stably positioned in the spring chamber to ensure that the abutting force applied to the relief valve by the spring remains the same and the relief valve can actuate more accurately; wherein the diaphragm is a flexible member being preferably made of a rubber material; and wherein the pump body is made of an easy-moldable plastic; characterized in that the exhaust chamber and the spring chamber are separated from each other; a valve cap is disposed over the pump cover and a buffer chamber is constructed by the pump cover and the valve cap; wherein the buffer chamber is communicating with the exhaust chamber; wherein a one-way valve is arranged at an air vent of the valve cap or the pump cover; wherein the one-way valve is open when air in the exhaust chamber is delivered to the outside, whereas, while the air in the exhaust chamber is not delivered to the outside, the one-way valve is closed.