Abstract: In order to explore the characteristics of pore size and porosity in low-rank coal reservoirs more accurately, and improve the mining efficiency of coal bed gas, based on the N MR experiments, this paper analyzed the characteristics of the peak time of T2 spectrograms, the distribution of micro-and meso-pores and effective porosity. The results show that NMR T2 sp ectrograms have three peak types. single-peak type, bimodal type and triple-peak type, the peak time of T2 spectrograms is at0.1 ~ 1.0 ms, about 10 ms and 70 ms, and coals pore size distri bution is mainly the micro-and meso-pores; the surface relaxation rate of low-rank coal is 1 .51x 10-8m/ms and the conversion coefficient C is 4. 53x 10-8m/ms, with which the NMR T2 spectrogr ams are transformed into pore size distributions of the micro-and meso-pores; The average effective porosity is 0.97% which is positive correlated with the fixed carbon, and negatively correlat ed with ash and volatile. Coal porosity is poorly connected, and the effective porosity is low, which is not conductive to coal bed methane exploitation.