Embodiments of the present disclosure provide a manufacturing method for a flexible device and a flexible display device. The manufacturing method for a flexible device comprises: step S1, forming an organosiloxane layer on a supporting substrate; step S2, forming a flexible substrate on the organosiloxane layer; step S3, forming a display device on the flexible substrate; step S4, performing an oxidation treatment on a surface of the organosiloxane layer that contacts the supporting substrate such that a silicon dioxide layer is formed between the organosiloxane layer and the supporting substrate; and step S5, peeling off the supporting substrate from the silicon dioxide layer.