The invention discloses a treatment method for treating solid waste with wastewater. Natural polymeric compound components are adopted for modifying the solid waste. The method includes the following steps: firstly, throwing the solid waste into water to obtain diluted water containing solid waste; secondly, dissolving a modifying agent into the diluted water to obtain water solution; thirdly, stirring the water solution, and standing prior to water solution layering with the upper layer being clear water phase and the lower layer being modified slurry; fourthly, dewatering the slurry on the lower layer by means of mechanical dewatering to obtain mud cake. The sludge is subjected to modification to jump out of the vicious cycle of dependence on the traditional desliming agent polymer, namely polyacrylamide (PAM), water content is lowered significantly in the dewatering link, energy consumption of follow-up sludge drying can be reduced, and fixed investment and other related operating cost are lowered. The invention further discloses a treatment device for treating solid waste with wastewater.