A method for forming thermal isolation for a micro thermopile device comprises steps of forming a narrow etching window on the membrane and forming a plurality of micro connection structures each crossing the narrow etching window and connecting the edge portion of the membrane on both sides of the narrow etching window, and etching the silicon substrate through the narrow etching window to form a pit between the silicon substrate and the membrane, whereby the membrane becomes a floating membrane and has thermal isolation with the silicon substrate. By this method, the area of the floating membrane is increased and the strain of the floating membrane is reduced.