Abstract: Acid filtration is one of the major factors that restrict the growth of acid etched fractures and filtration control is the key to deep penetrating acid fracturing technology. This paper summarizes the characteristics of acid filtration and the development history of acid filtration control technologies, with focus on the variable- viscosity acid technology that was introduced and put into field application at the end of 1990s. Practices both at home and abroad show that compared with the widely used gelling acid fluid system at present, the effectiveness of variable - viscosity acid is higher and the effect of stimulation is better. The introduction and advancement of this technology will surely accelerate the development of domestic deep penetrating acid fracturing technology.