The pores in reservoirs bearing hydrocarbon have been divided into three types：millimeterpore，micropore and nanopore. The diameter of pore throat in conventional reservoir is generally larger than1 μm. The diameter of pore-throat in tight gas reservoirs ranges from 0.03 to 1 μm. The nanopore is the main body of the connected reservoir space in tight sand，so it is difficult to evaluate the effectiveness of tight reservoirs. Both NMR T2 spectrum and capillary pressure curves can well reflect the pore throat structure of rocks. The NMR T2 spectrum was translated into pore throat distributions using the correlation between NMR T2 experiment and mercury injection. On the basis of previous research results，the core NMR T2 spectrum and capillary pressure data were processed and analyzed deeply to determine effectiveness criteria of the tight sand pore throat in SLG oilfield：the pore-throat system with pore radius less than 0.04 μm is associated with clay bound water volume；the pore-throat system with a pore radius between 0.04 and 0.1 μm is associated with non-shale micro-porosity formation water volume；the pore-throat system with pore radius between 0.1 and 0.2 μm is associated with capillary bound water volume；the pore-throat system with pore radius more than 0.2 μm is associated with the volume in which the fluid can be taken out. The practice proves that the method can be used to quickly calculate pore-throat space quantitatively and to determine the pore water features and its occurrence，which can evaluate the space effectiveness of tight sand reservoirs effectively.