The online monitoring of fluids flowing status in cores and visual evaluation had become important experiment method for oil displacing and enlarge sweep volume mechanism. In this paper， low field nuclear magnetic resonance and core flooding equipment were combined， and the distribution of transverse relaxtion time（T₂）was obtained based on nuclear magnetic resonance theory. Combined with mercury injection pore radius analysis results， the relationship between nuclear magnetic resonance T₂ spectrogram and pore radius of core was established using interpolation and least square method. Taking two kinds of cores with medium-high permeability（256 × 10^-3 μm²）and low permeability（7.51 × 10^-3 μm²）as an example，binding with core flooding experiments of different fluids，and using conversed nuclear magnetic pore throat distribution，the relationship between pore structure of core and the distribution of movable fluids and residual oil was obtained. The oil exploiting rate in different pore by water flooding and polymer flooding was also analyzed. The results showed that the average pore diameter of medium-high permeability core was 72 μm， the pore diameter was mainly 1—500 μm， indicating good micro-level pore development. The average pore diameter of low permeability core was 86 nm，the pore diameter was mainly 10 nm—1 μm，indicating good nano and sub-micro-level pore development. The bound water of medium-high permeability core and low permeability core was mainly concentrated in the pore space with the diameter less than 1 μm and 0.5 μm respectively. The core flooding experiments results showed that oil in pores larger than 10 μm was displaced by water， and polymer flooding showed the same tendency and oil flooding efficiency was limited. More than 80% of the residual oil was in the pores smaller than 1 μm. Improving the water flooding efficiency of low permeability zone was the key point for EOR in the future.