The invention discloses a compound heavy metal adsorbent. The compound soil heavy metal adsorbent comprises a floating core and a compound high molecule coating the floating core, wherein the mass ratio of the floating core to the compound high molecule is (1-5):1; the compound high molecule comprises a first polymer and a second polymer of which the mass ratio is (5:1)-(1:5); the first polymer is alginic acid or calcium alginate; the second polymer is modified chitosan; a modification group of the modified chitosan is small molecule chelator; the small molecule chelator has a carboxyl group and at least two chelation groups; the amino group ratio of the small molecule chelator substituted chitosan is above 30%; and the remaining amino groups in the chitosan and hydroxyl groups of the first polymer are crosslinked through dialdehyde molecules. According to the invention, the soil heavy metal adsorbent is prepared by compounding the modified chitosan modified with the small molecule chelator with the alginic acid/calcium alginate and the floating core, thereby improving the adsorbability of the soil heavy metal adsorbent.