The invention relates to a method for efficiently preparing cotton based mesoporous activated carbon fiber. The method comprises a step (1) of washing cotton, placing the cotton in a hydrothermal reaction kettle, performing heating to 150-200 DEG C at the heating rate of 5-30 DEG C/min, performing hydrothermal carbonization, performing heat preservation for 1-3 hours and then performing cooling to room temperature; a step (2) of dipping carbonization materials obtained from the step (1) into activator solution, performing microwave-ultrasonic joint dipping for 5-30 minutes and then performing filtration, sending filtration materials to a microwave heating device, performing activation for 5-15 minutes at 250-350 DEG C and then stopping microwave output, and a step (3) of washing the activated materials to be neutral and performing drying to obtain the activated carbon fiber. According to the method, the characteristics of microwave inner heating and selective heating are fully utilized, the technological process is simple, the activation temperature is only 250-350 DEG C, the activation time is only 5-15 minutes, the mesoporous proportion of the activated carbon fiber of the product reaches 50-90%, and the adsorptive property is good.