The invention provides an active clamp flyback converter self-adaptive control method and circuit, which are used for controlling a conduction time of a clamper tube. The active clamp flyback converter self-adaptive control method and circuit have the following characteristics and effects: a multiplexing auxiliary winding voltage sampling pin determines whether a main switch pipe ZVS is switched on or not, and no additional pin is needed, so that the encapsulation size is reduced; a switch node voltage proportional value in the conduction period of the main switch pipe is saved, and the conduction realization situation of ZVS is determined cycle by cycle on a rising edge of a main switch pipe driving signal, so that the self adaption can be actually realized; since the saved switch node voltage proportional value is gradually updated cycle by cycle, the determination result of the conduction realization situation of the main switch pipe ZVS is not influenced by the device precision, and the determination result is more accurate; and by virtue of the bidirectional control for the width of the clamper pipe driving signal, on the premise of realizing the conduction of the main switchpipe ZVS, the negative current flowing by a diode of the main switch pipe body can be reduced as far as possible, and the loss can be reduced.