Flow control using plasma actuator is a promising research field of aeronautical applications. Due to its low energy consumption, rapid response and simple construction, this actuator has been investigated in various aerodynamics problems, such as boundary layer flow control, drag reduction, lift enhancement, noise reduction, and flow separation control. In order to understand the controlling mechanism of plasma actuator, many researchers have been carried out some experiments on the plasma actuator characterization in quiescent air and obtained the evolution process of starting vortex induced by plasma actuator. But the plasma actuator always works under flow condition. Therefore, understanding the interaction process between the starting vortex and incoming flow is a key to promote this technology development. In this paper, the starting vortex induced by symmetrical Dielectric Barrier Discharge (DBD) plasma actuator in quiescent air or under flow condition was investigated using Particle Image Velocimetry (PIV). Compared with the asymmetrical DBD plasma actuator, the symmetrical plasma actuator adopted the whole metal plate model as the insulated electrode. Three layers of kapton film as dielectric material covered the testing model and the thickness of each layer was 0.05 mm. The copper foil which was 2 mm in width and 0.05 mm in thickness was mounted on the trailing edge of the plate and oriented along the spanwise direction to induce a wall jet in the streamwise direction. The input AC voltage was 8 kV p-p and the frequency of the power source was 3 kHz. The wind speed was 1 m/s. The results suggested that the symmetrical actuator produced one pair of counter-rotating starting vortexes on each side of upper electrode and the trajectory of the starting vortex core was shown to scale with t0.7 in quiescent air. Compared to the evolution law of starting vortex in still air, the development evolution and life time of starting vortex under flow condition was different due to the interaction influence between incoming flow and starting vortex. The breakdown time of downstream starting vortex was earlier and the location of the starting vortex core scaled with t0.45 under flow condition. Conversely, the life time of upstream starting vortex which was in the opposite direction of incoming flow was delayed. The incoming flow enhanced the upstream starting vortex's capability of promoting mixing and entraining high-momentum fluid into boundary layer, therefore the boundary layer became more energetic and capable of withstanding adverse pressure gradient. The jet effect and mixing function could be achieved by the symmetrical plasma actuator. These investigations laid the groundwork for flow control using DBD plasma actuator at high wind speed or high Reynolds number.