Abstract: The existing underwater synthetic jet actuators mainly adopt mechanical transmission mode such as cam mechanism, the mechanical structure is complex, and the stroke is not adjustable. For these problems, an electromagnetic underwater synthetic jet actuator with simple mechanical structure and adjustable stroke is designed. The electromagnetic actuator produces synthetic jet by the piston in a straight line reciprocating motion driven by voice coil actuator. A series of thrust experimental studies for the actuator are carried out under different cavity nozzle diameters, different piston vibration frequencies and different piston displacements. The experiments show that the synthetic jet average thrust is inversely proportional to the nozzle diameter and proportional to piston vibration frequency and piston displacement. The efficiency of the actuator increase when the nozzle diameter increases. The results show that the electromagnetic underwater synthetic jet actuator can produce the average thrust, which is a novel type of underwater robot attitude adjustment.