The invention discloses a method for preparing composite dephosphorus adsorbent by virtue of activated carbon fiber loaded with metal. Carbon fiber is used as a matrix which is loaded with a metallic compound. The method comprises the following steps of: a, mixing lanthanum ions into an iron salt for preparing a mixture solution of iron ions and lanthanum ions; b, putting the prepared solution under an ultrasound environment, additionally adding a radiation method, gradually adding a hydroxide solution into a metallic ion mixture solution until the pH reaches 9-10, and obtaining nano-particle suspension liquid of iron and lanthanum composite metal hydroxide; c, shearing activated carbon fiber into pieces, washing, dipping, filtering and drying; d, weighing the activated carbon fiber, adding the activated carbon fiber into the suspension liquid, processing by utilizing ultrasound waves, taking out solid matters, and washing for multiple times until the pH reaches 7; and e, drying the solid matters, obtaining an adsorbent material after cooling, and sealing the adsorbent material for storage. According to the invention, the iron and lanthanum composite metal hydroxide is in uniform particle size distribution on the carbon fiber matrix, has a high adsorption capacity and a low preparation cost and is suitable for industrial application.