The invention belongs to the technical field of gas separation, and particularly relates to a method for effectively separating CO2 in flue gas by a modified activated carbon nanosheet. An ultrasonic peeling method is mainly adopted to obtain an activated carbon nanosheet; then, an acid solution is used for carrying out oxidative modification; finally, a breakthrough experiment indicates that the separation volume of a modified activated carbon nanosheet material subjected to vacuum activation at a temperature of 80DEG C for CO2/N2 mixed gas (v to v=15 to 85, and total flow is 20mL/min) at room temperature is 15.79cm<3>g, and separation selectivity is 4.37. The modified activated carbon nanosheet can be used as a solid adsorbent material for effectively separating the CO2 in the flue gas.