The adaptive synchronous rectification control circuit and its controlling method for a power converter are provided. The proposed control circuit having a transformer with a primary and a secondary sides, a switch coupled to the primary side and a synchronous rectification transistor (SRT) coupled to the secondary side and having a parasitic diode includes a preprocessor receiving a source-drain voltage of the SRT and outputting a first signal, and a control set receiving the first signal, a pre-determined voltage and a pulse signal synchronized to the switch and generating a second signal synchronized to the switch for controlling the SRT, in which a selection of the pre-determined voltage makes the source-drain voltage of the SRT ranged from 0 to a conducting voltage of the parasitic diode of the SRT.