Owing to high-turbulent and shear flow, co-current air-water jets can serve as bubble generators to produce fine bubbles efficiently. In the process of bubble generation, the concentration of bubble swarm is detected by the imaging grayscale obtained by a high-speed camera. We report a detailed experimental characterization of the instability of the concentration of bubble swarm at a jet outlet. To suppress this instability, a novel bubble generator, which consist of two or three serial jets with the same geometrical dimension, was investigated. Current study shows that for a certain of liquid Reynolds number, serial jets increase the stability and improve the mono dispersion of bubble size.