A portable warm bag having steam heating device includes a rectangular bag having a pressure sensor, a pressure releasing valve, a switch bottom and a valve cover on both sides of the upper lid, a partition vertically disposed in the bag adjacent a lateral side thereof to define a gap therebetween to receive a vapor creater therein which supplies the vapor into the food containing space via a hose and through a perforated plate above the bottom of the bag, a power source compartment under the bottom of the bag containing a plurality of rechargeable batteries therein for heating a heating bar in the vapor creater to boil the water to create vapor to into the bag to establish a heating circulation into the food which is kept at constant temperature eatable to the user.