The invention discloses an absorbent used in sewage phosphorus removal. A carrier of the absorbent is industrial lithium-silicon powder waste slag with a size no lower than 150 meshes. Lanthanum ions are loaded on the carrier. With the absorbent, the removal rate upon phosphorus in phosphorus-containing sewage and wastewater or natural water body with a total phosphorus concentration of 1mg/L and a pH value of 3-8 is always higher than 86%. The invention also discloses a preparation method of the absorbent. According to the invention, the carrier raw material lithium-silicon powder adopted by the absorbent is selected from industrial waste slag, such that the source is wide, and cost is low. With a developed pore structure, the carrier can provide sufficient active absorption points. Only trace of rare earth lanthanum used for modification is needed to be added. The obtained absorbent has excellent phosphorus absorption effects in natural water bodies, sewage and wastewater. With the absorbent, an absorption capacity is high, absorbent stability in water body is high, no water body secondary pollution is caused, and cost is low. The absorbent has a good application prospect.