Abstract: When drilling deep fractured formation, the formation breathing effect caused by wellbore pressure fluctuation is easy to be misjudged as overflow and unnecessary killing measures are taken, which seriously affects the drilling safety. Therefore, an experimental equipment was developed to simulate the breathing effect of fractured formation. A series of experiments were carried out to analyze the effects of drilling fluid circulation pressure, fracture opening pressure and rock type on the characteristics of drilling fluid leakage and flowback. The results show that the fractured breathing effect is a reversible leakage, and the whole process of its occurrence is leakage-“quasi-steady”-flowback. With the increase of circulating pressure of drilling fluid, the maximum leakage and return amount of drilling fluid increased. With the increase of fracture opening pressure, the maximum leakage amount of drilling fluid increases, but the return amount decreases. Compared with sandstone and limestone, granite with extremely low porosity and low permeability has larger leakage and smaller return amount. The research results provide a theoretical basis for analyzing the characteristics of fractured breathing effect of deep fractured formation and adopting correct measures to distinguish other operating conditions.