Abstract: Coal seams in China generally have the characteristics of low permeability and high gas content. To improve the permeability of coal seams,water infusion,hydraulic fracturing,hydraulic slotting and other hydraulic technologies have been extensively used,which have good effects on gas control. However,When water enters the coal seam as an extravasation fluid,it blocks the gas flow channel,and reduces the volume of gas desorption,which results from the wa- ter blocking effect. In order to analyze the internal mechanism of water blocking effect caused by hydraulic technology, the pore volume distribution of coal samples was analyzed by mercury injection experiment,and the microstructure of raw,water saturated and CMC solution saturated GQ coal samples were analyzed by scanning electron microscope. The liquid phase retention effects of different coal samples saturated by water and CMC solution were studied by low field nuclear magnetic resonance (NMR) technique. According to the curve similarity method,the similarity between pore size and irreducible fluid saturation was analyzed. The results show that CMC solution can dissolve the minerals in coal matrix,which increases the coal pore fissure,and reduce the surface tension of water on the coal surface,which proves that adding CMC can relieve the water blocking effect. With the increase of coal metamorphism,the T2 cutoff value de- creases gradually,and the T2 cutoff value is negatively correlated with the pore size of coal sample,the irreducible fluid saturation of coal samples are much higher than that of free fluid saturation,and the irreducible fluid saturation of coal samples are higher than that of saturated CMC solution. The coal with high metamorphic degree has less macroporous volume and more micropore volume,which makes the capillary force larger,leading to serious water blocking effect in high metamorphic degree coal. The macroporous volume is the main control factor affecting the irreducible fluid satura- tion,and the micropore plays a positive role in promoting the irreducible fluid saturation. The coupling relationship be- tween the irreducible fluid saturation and the macroporous volume and the micropore volume is obtained as S = 94． 86-1 078． 96VA +261． 24VD . Retention of irreducible water in the coal blocks the gas flow channel,which is the root cause of water blocking effect. Increasing the pores and cracks in the coal seam and selecting the appropriate surfactants are the effective measures to relieve the water blocking effect of coal seam.