The influence of speed and air flow on the flat gas reservoir law of drag reduction was investigated by an experiment of plate with air injection, and the air layer formation in different groove configuration was observed by an underwater imaging system. The results show that the grooves increase the resistance of the plate without air; the resistance of flat plate can be reduced with the injection of air, and there is an optimal air flow; the optimal air flow decreases with the rise of the groove depth, and the absolute drag reduction rate decreases with the increase of the speed; the configuration of the groove is an effective measure to maintain the stability of the plat bottom surface air layer; the appropriate groove configuration significantly improves the flow field at the bottom of the flat plate and increases the effect of the flat drag reduction with air injection; the absolute drag reduction rate reaches more than 40% under the optimal air flow when the Fr is 0.119.