The invention belongs to the technical field of microelectronic and flat panel display, and particularly relates to a flexible ZnO based thin film transistor and a preparation method thereof. The flexible thin film transistor comprises a flexible substrate, a flat layer, an isolation buffer layer, a gate, a gate insulating layer, a source electrode, a drain electrode and semiconductor channel layers. N type and P type zinc oxide semiconductor channel layers are prepared by using a simple inorganic solution method, and the temperature of the whole process is controlled within 200 DEG C. When in manufacturing, the flat layer and the isolation buffer layer with the same thickness are grown at two sides of the substrate, the flexible substrate bending in the process is alleviated through a thermal annealing mode, and the subsequent lithography alignment accuracy is improved. According to the transistor and the preparation method, the maneuverability and stability of the flexible TFT and subsequent devices can be improved, the preparation process is simple, and the production cost is low.