A self-wringing flat mop, comprising a flat mop head further comprising a central plate and two side plates, wherein two sides of the central plate are respectively hinged with the two side plates via torsion springs, and wherein the central plate includes a connection through-hole formed at the center thereof; a cleaning part whose two ends are respectively assembled to the two side plates, including an insertion-connection member corresponding to the connection through-hole; a mop handle further comprising a connection rod assembled to the central plate, a holding rod disposed coaxially with the connection rod, and a transmission assembly disposed between the connection rod and the holding rod, wherein the connection rod includes a control member penetrating the connection through-hole and connected with the insertion-connection member, and wherein the holding rod is configured to be operated to generate a rotation, and the transmission assembly is configured for transmitting the rotation to the connection rod, and wherein the connection rod turns the cleaning part through the control member to make the two side plates collapse and wring water out of the cleaning part, and wherein the transmission assembly includes a first engaging member and a second engaging member, and wherein the first engaging member normally contacts the second engaging member, and wherein a rim of the first engaging member has a guiding member, and wherein the holding rod includes a limiting rib; and a holding sleeve sleeving the mop handle and assembled corresponding to the transmission assembly, characterised in that said holding sleeve further comprising a control lever pivotally installed with respect to the guiding member, and a unidirectional limiting ring corresponding to the limiting rib, wherein the control lever includes a protruding trigger protruding from the holding sleeve and contacting the guiding member, and a levering element connected with the protruding trigger, and wherein the levering element is turned pivotally with respect to the holding sleeve to make the protruding trigger slide on the guiding member and separate the first engaging member from the second engaging member, and wherein the unidirectional limiting ring includes a plurality of ratchet teeth arranged annularly and facing the mop handle, and wherein the ratchet teeth confine the limiting rib to slide unidirectionally and confine the mop handle to turn in a single direction. a flat mop head further comprising a central plate and two side plates, wherein two sides of the central plate are respectively hinged with the two side plates via torsion springs, and wherein the central plate includes a connection through-hole formed at the center thereof; a cleaning part whose two ends are respectively assembled to the two side plates, including an insertion-connection member corresponding to the connection through-hole; a mop handle further comprising a connection rod assembled to the central plate, a holding rod disposed coaxially with the connection rod, and a transmission assembly disposed between the connection rod and the holding rod, wherein the connection rod includes a control member penetrating the connection through-hole and connected with the insertion-connection member, and wherein the holding rod is configured to be operated to generate a rotation, and the transmission assembly is configured for transmitting the rotation to the connection rod, and wherein the connection rod turns the cleaning part through the control member to make the two side plates collapse and wring water out of the cleaning part, and wherein the transmission assembly includes a first engaging member and a second engaging member, and wherein the first engaging member normally contacts the second engaging member, and wherein a rim of the first engaging member has a guiding member, and wherein the holding rod includes a limiting rib; and a holding sleeve sleeving the mop handle and assembled corresponding to the transmission assembly, characterised in that said holding sleeve further comprising a control lever pivotally installed with respect to the guiding member, and a unidirectional limiting ring corresponding to the limiting rib, wherein the control lever includes a protruding trigger protruding from the holding sleeve and contacting the guiding member, and a levering element connected with the protruding trigger, and wherein the levering element is turned pivotally with respect to the holding sleeve to make the protruding trigger slide on the guiding member and separate the first engaging member from the second engaging member, and wherein the unidirectional limiting ring includes a plurality of ratchet teeth arranged annularly and facing the mop handle, and wherein the ratchet teeth confine the limiting rib to slide unidirectionally and confine the mop handle to turn in a single direction.