Abstract: During hydraulic fracturing in coal seams， fractures expand to coalrock interface and the direction of fracture growth may change because of the effects of many mechanical factors. Based on the 2D model of coalrock interface in hydraulic fracturing， the theoretical analysis and numerical simulation were conducted to investigate the failure mechanism of coalrock interface and the propagation characteristics of fractures induced by hydraulic fracturing. It is shown that:the main factors which affect the development direction of fractures are intersecting angle between coalrock interface and horizontal profile， horizontal stress difference， minimum horizontal insitu stress， cohesive strength of coalrock interface and elastic modulus difference between coal and rock. With the decrease of intersecting angle or horizontal stress difference， the probability of fractures crossing coalrock interface will raise; while the minimum horizontal insitu stress increases， the probability of fractures crossing coalrock interface will reduce; with the decrease of cohesive strength of coalrock interface or increase of elastic modulus difference between coal and rock， the fractures will tend to extend along the coalrock interface.

Key words: hydraulic fracturing, fractures propagation, coal seam, coalrock interface, internal critical water pressure