Abstract: The Bionic Robofish Technology is one of the hotspots in the underwater robotics research field in recent years. It provides a new train of thinking for people to develop the underwater vehicles with high efficiency, high maneuverability, and low noise. This article makes some research on the fish vertebral curve, brings forward a fish propulsion theory-undulate propulsion. And the drag during the fish's undulate propulsion is also analyzed. Through the observing experiments on real fish swimming and bionic robot eel developing, we prove the efficiency of this theory.