Performance of a two-electrode plasma synthetic jet actuator has been experimentally studied by discharge measurements with high-speed shadowgraphy technology. Results show that the breakdown voltage and the peak discharge current of the actuator may be decreased by decreasing the ambient pressure and increasing the discharge frequency. The discharge developed in the actuator cavity is a spark-arc discharge. In the actuator could be created a strong “precursor shock” and a high kinetic energy jet. During the development of the plasma synthetic jet, the speed of the “precursor shock” is invariable and the jet propagates with an approximately local sonic velocity (350 m/s). But with decreasing jet exit diameter and ambient pressure, the increase of the cavity volume and the discharge frequency could lead to decreasing strength of the “precursor shock”. Heating efficiency of the gas in the cavity will decrease with the increase of the cavity volume and discharge frequency, and the jet velocity is decreased as well. The jet exit diameter and the ambient pressure can have their optimal values for affecting the velocity of the jet. Under all the working conditions we have studied, the plasma actuator may create a strong “precursor shock” and a high-speed jet, and also may have the potential to be used in high-speed active flow control.