A method of fabricating a polymer-based capacitive ultrasonic transducer, which comprises the steps of: (a) providing a substrate; (b) forming a first conductor on the substrate; (c) coating a sacrificial layer on the substrate while covering the first conductor by the same; (d) etching the sacrificial layer for forming an island while maintaining the island to contact with the first conductor; (e) coating a first polymer-based material on the substrate while covering the island by the same; (f) forming a second conductor on the first polymer-based material; (g) forming a via hole on the first polymer-based material while enabling the via hole to be channeled to the island; and (h) utilizing the via hole to etch and remove the island for forming a cavity.