Abstract: Whether the residual acid is flowed back or not after acidization is very important to the stimulation effects. gas is a better assistant for residual acid back flow because of its easiness to compress and expansion after depressurized. The functions of inversion, separation, filtration decrease, back flow assistant of Nitrogen gas are used to strengthen itself back flow capability of factory residual acid, to improve back flow speed and extent in the Gas mixed acidization technology. Therefore the goals of formation protection and production improvement with formation cleaning can be reached. It specially meets the need to treat the poor permeability, low pressure and high sensitivity formations. The paper gives a brief introduction of the gas injection method, operation processes and operation characteristics and the influence factors, which affects the backflow volume. The result shows that this technology is simple and applicable. The effect of back flow assistant is better.