Abstract: Using the discrete element numeric simulation program UDEC,the evolution law of mining-induced fractures in the mining process in steep coal seam was studied. Combined with the study of similar material simulation experi- ment results,the result by numerical calculation was verified. The results indicate that bed separation fissures are the main type at the initial stage,the height of the bed separation fissures remain constant and the vertical fissures speed up to extend in the later period. When the vertical fissures extend to the surface,ladder cracks and collapse pits may be formed. The relational expression between fractal dimension (D) of the mining-induced fracture and mining depth (H) was established,and the relational expression between percolation probability (p) of the mining-induced fracture and mining depth (H) was also established based on the numerical simulation,so that the evolution law of mining-in- duced fracture can be quantified. By fitting the relation between the fractal dimension (D) and penetration probability ( p),the peak value of fractal dimension can be obtained and the corresponding mining depth,which is needed for re- forming the stable structure of broken rock masses along the dip direction,can also be obtained. The study can be used to estimate whether steep coal seam can form a stable main roof structure in the range of mining depth in advance,and then guide the rational support measures of roadway and the safe mining of working face.