The tight limestone in Jurassic Daanzhai Formation of Central Sichuan Basin is one of the most important tight oil reservoirs in China. The Daanzhai Formation is considered as ultra-low porosity and permeability reservoir with nanometer and micrometer pores and fractures. A total of thirty core plugs were saturated with saline water for days，and then nuclear magnetic resonance（NMR）T2 measurement was conducted. The results obtained show that there are four types of peak distribution patterns for NMR T2 spectrum，including unimodal distribution，bimodal distribution（dominated either by left peak or right peak）and pseudo triple-modal distribution. It is found that core samples with unimodal NMR T2 spectrum are characterized by nanometer pores with low permeability through comprehensive analysis of the statistic relationship between pore components and reservoir permeability and the microscopic characteristic of pores. Although T2 spectrum distribution shifts to short relaxation time，the permeability value can be high with a good relationship between components of larger pores and flowing capacity when connected intercrystal pores，extended intercrystal fractures and inter-layer pores within clay minerals develop well. When the T2 spectrum distribution shifts to long relaxation time，the permeability value can be low with a weak relationship between components of larger pores and flowing capacity if inter-shell and intercrystal pores are locally developed and intercrystal fractures or inter-layer fractures within shells has limited length. The micronscale fractures such as structural fractures and intercrystal fractures contribute to the increase of reservoirs permeability obviously.