The invention belongs to the technical field of flow sensors, and discloses a flow sensor based on a polymer and a preparation method of the flow sensor. The flow sensor sequentially comprises a support layer, a flexible substrate, a thin-film resistance layer and a flexible covering layer from bottom to top, wherein the thin-film resistance layer comprises a heating resistor, a downwind direction measurement resistor and an upwind direction measurement resistor; a groove is formed in one side, facing the flexible substrate, of the support layer, and the groove is right under the heating resistor, the downwind direction measurement resistor and the upwind direction measurement resistor. According to the flow sensor, the flexible substrate serves as a carrier; the thermal conduction is reduced as the support layer with the groove is arranged under the substrate; as the flexible substrate serves as the carrier of the flow sensor, the strength of a thin-film structure on the surface of the sensor is effectively improved, and the thin-film structure is not prone to being broken or collapsed; furthermore, the flow sensor is low-temperature-resistant and acid-resistant, and the application range of the flow sensor is greatly enlarged. According to the flow sensor, a traditional silicon substrate is replaced by the polymer support layer, the processes of growing and etching silicon nitride are eliminated, and the technological difficulty and the cost are reduced.