The invention discloses a preparation method of ceramic additive by modified composite biomass base and an application thereof. The method includes steps of mixing concentrated landfill leachate with starch under ultrasound wave catalyst, and then preparing black loose powder shaped ceramic additive through alkalization, sulfonation and quaterisation, and performing spraying and drying; after mixing, performing alkalization and sulfonation reactions on landfill leachate concentrated solution and starch; thus the activity of effective molecular structure can be improved; the further quaterisation can effectively control the product molecular weight, thus the prepared ceramic additive has good dispersing performance while can largely improve the ceramic blank generating intensity; moreover, the prepared ceramic additive has multiple functions such as dispersing, water reduction, wear assisting, sticking and defoaming; thus the side product of the waste treatment is recycled and effectively used, the crisis of non-renewable energy resource is released, the actual application value and the social value of the side product during the waste disposal process are realized.