Abstract: A wave-vortex principle of oscillating propulsion is presented,which is based upon the observation and experimental study of fish′s oscillating propulsion.The theoretical analysis shows,that the occurence and interaction of 8-shaped vortex is the reason for creation of thrust.The experiments accomplished with oscillating stiff blades give a strong heuristic evidence.It is that the discovered 8-shaped vortex possesses behaviour of energy storage.This new principle is of great significance for analysing fish′s swimming and designing high efficiency propeller.