A self-wringing flat mop includes a flat mop head, a mop handle and a holding sleeve. The mop handle includes a connection rod, a holding rod, and a transmission assembly. The holding rod is operated to rotate. The transmission assembly transmits rotation to the connection rod. The connection rod drives the flat mop head to collapse and wring out water. The transmission assembly includes a first engaging member and a second engaging member for undertaking engagement-disengagement actions. The holding rod includes a limiting rib. The holding sleeve sleeves the mop handle and includes a control lever. The control lever is turned pivotally with respect to the holding sleeve to separate the first engaging member from the second engaging member and restore the collapsed flat mop head to the flattened state. The holding sleeve includes a unidirectional limiting ring corresponding to the limiting rib.