Synthetic jet is a new type of underwater propulsion method with the merit of small size,light weight and high efficiency. All of which makes it a good choice for the actuation and steering of underwater robots. The forcing profile of the synthetic jet actuator has a great effect on its average thrust. In order to study the working mechanism of the effect of the forcing profile on the thrust characteristics of the synthetic jet actuator,the finite volume numerical method is applied to analyze the 3D,unsteady,viscosity and incompressible flow-field of a synthetic jet actuator. The forcing profile is set to be of sinusoidal type with different frequency during injection and ejection period. It can be seen from the numerical result that the decrease of the frequency of either of the injection or the ejection period will bring down the average thrust of the synthetic actuator. It can also be seen that the direction of the average thrust can’t be reversed by the frequency change of the injection and ejection period. The mathematical model of the thrust is established which can be used to explain these characteristics.