A portable air pump includes an inflating mechanism, a joint seat disposed on a cylinder of the inflating mechanism, a support seat pivotally coupled to the joint seat, a positioning mechanism pivotally coupled to the support seat, and a footrest is coupled to the support seat. The joint seat has a joint end including a positioning portion and a groove. The support seat includes a connection pivotally received by the groove and has a receptacle. The positioning mechanism includes a positioning member pivotally engaged in the receptacle and a biasing member. The positioning member is biased by the biasing member and detachably engagable with the positioning portion. The footrest is folded out when the positioning member is engaged with the positioning portion and adapted to be folded toward the inflating mechanism when the positioning member is disengaged from the positioning portion.