Abstract: In order to increase gas drainage efficiency of coal seam and improve the permeability of coal reservoir, the coal sample was drilled to inject liquid nitrogen and scanned by C T, so that the fracturing process and the permeability effect were analyzed. The results of study show that affected by double action of the cold shock shrinkage stress and the pore pressure, t he fractures appear around the boreholes which has been injected liquid nitrogen, the porosity variation of coal has a negative exponential decrease with the increase of the distance from the borehole .With the increasing distance from the borehole, the fracturing effect of liquid nitrogen injection on coal weakened;due to the broken expansion, the area near the borehole has a bette 「effect of permeability improving, while the effect on the area far from the hole is converse .There were some correlations between the fracturing effect on the coal sample and its original poro sity: in the area of lower original porosity, the porosity increases more obviously, while in the area of higher original porosity, the increasing of porosity is not obvious or even decreased.The re sults show that nitrogen injection can improve the permeability of coal reservoir and increase the output of coalbed methane production.