To study influence of diffusion coefficient, relaxation rates, pore component ratio and pore width on NMR T₂ distributions, a complex rock-pore system is simplified to an equivalent dual porosity medium in a mathematical physics model with Bloch's equation. ADI-FDTD and SIRT methods are used in numerical simulations of NMR transversal response and decomposition of NMR T₂ spectrum, respectively. It shows that transversal magnetization vector decay rate is proportional to diffusion coefficient and micro porosity, inversely proportional to pore width and nearly irrelevant to surface relaxation rate. NMR T₂ spectrum hardly reflects pore components and structure as diffusion coefficient is small and pore width is large. Special attentions should be paid to influence of diffusion coefficient and pore-size as evaluating pore structure by NMR.