A portable constant temperature box is formed of a box body and a lid, the box body being formed of a square bottom wall of adiabatic material, four side walls, and an external sheet. The side walls are connected to the bottom wall such that they can unfold and rise and the external sheet forms foldable connecting portions for connecting the side walls. A pair of retainers fix the connecting portions detachably to the side walls to thereby hold the side walls to rise upright respectively.