The invention discloses a bagasses based adsorbent with a high grafting rate, and a preparation method and an application thereof. The preparation method comprises the following steps of (1) pretreatment of the bagasse: smashing and screening the bagasse, boiling 20-30 meshes of bagasse with water, washing the bagasse clean, filtering by suction, immersing with a certain concentration of an alkaline solution, and then carrying out pretreatment on bagasse fibers through steam explosion and surface activation processes; (2) graft modification of the bagasse: carrying out a grafting reaction at a certain temperature by using Mohr's salt/hydrogen peroxide/acetic acid as a compound catalyst; and (3) aftertreatment of modified bagasse: washing a resultant with water, stewing with boiling water, washing with deionized water, filtering by suction and drying. The preparation method provided by the invention is simple, has a high grafting rate, increases the content of grafting functional groups on a bagasse based adsorbent material, and greatly increases an application effect of the bagasse based adsorbent material in the field of heavy metal absorption.