Abstract: In order to study the relationship between natural fractures and artificial fractures induced by fracturing in coal reservoir in Lu'an mining area, macro-fractures were measured underground and micro-fractures were observed under microscope. The fracturing law of heterogeneous coals and rocks with fractures was discussed, the critical angels between the direction of maximum horizontal principal stress of artificial fractures extending along natural fractures in coal reservoir and the direction of natural fractures were calculated. The results showed that the critical angels of five collieries are 12.00°, 9.22°, 9.16°, 8.56° and 7.42° respectively. Artificial fractures will extend along the direction of natural fractures when the angel between the two directions is less than the critical angel. Otherwise, it will extend along the direction of maximum horizontal principal stress. Based on the above research, three types of artificial fracture extension along the natural fractures were concluded.