The invention discloses a system and method for treating heavy metal in excess sludge through water hyacinth. The system comprises a reaction pool used for allowing the water hyacinth and the excess sludge to react and a sludge discharge pool; the bottom end of the reaction pool is inclined to the horizontal plane, and the distance between the bottom end of the reaction pool and the horizontal plane is larger than the that between the bottom end of the sludge discharge pool and the horizontal plane; one side of the reaction pool is provided with a sludge inlet, and the other side of the reaction pool is provided with a supernatant outlet and a sludge discharge pipe connected with the top of the sludge discharge pool; the bottom end of the reaction pool is provided with an oxygen filling stirring device. The method comprises the step that treatment on the heavy metal in the excess sludge is achieved through the system for treating the heavy metal in the excess sludge through the water hyacinth. Through treatment of the water hyacinth reaction pool, the treatment effect on the heavy metal in the excess sludge is good.