In order to investigate the applicable range of different CO₂ flooding methods in reservoirs with micron/Nano sized pore-throat, a new evaluation method has been found by and high temperature high pressure nuclear magnetic resonance. The conversion coefficient of NMR T2 relaxation time and pore throat radius was established by using the pore throat distribution data obtained, and the oil displacement range and source of enhanced oil were described by formation-core driving experiments. Experimental results show that: water can enter 0.1μm scale pore-throat,CO₂ can enter 0.01μm scale pore-throat, water drive oil was mainly movable oil and CO2 flooding increased oil was bound oil. Core macroscopic throats was formed after multiple displacement. The water alternating gas flooding(WAG) and foam flooding could produce block in macroscopic throats, but the adapt of foam flooding was wider than WAG. Below the level of 0.1um pore throat, the recovery degree of foam flooding is higher than WAG 30 percentage points.