Abstract: To study the hydraulic fracturing mechanism and its dynamic characteristics，taking 12th mine of Pingdingshan Coal Group as example，a related mathematic model was built，and the dynamic characteristics of fracture initiation and extension were gained in the progress of hydraulic fracturing by numerical simulation and field test，which is described as: stress cumulating →micro-cracks steadily developing →partially destroyed area forming →partially destroyed area extension and coalescence →cracks unsteadily developing．Combined with the analysis of onsite measured data and influencing factors of hydraulic fracturing dynamic changes of coal body containing gas，the coupling model which is composed of coal seam depth，gas pressure and hydraulic fracturing pressure is fitted．The results have an important significance to the field practice．

Key words: hydraulic fracturing, fracture mechanism, dynamic characteristics, numerical simulation