The invention discloses a biomass asphalt modifier and a preparation method and application thereof. The biomass asphalt modifier comprises the following raw materials: rice straw, a liquefying agentand a catalyst. Rice straw, which is used as a raw material of the biomass modifier, is subjected to a chemical reaction with a mixture of polyethylene glycol 400, glycerol and p-hydroxyphenylethyl alcohol in a high-temperature and high-pressure environment under the catalytic action of acid so as to prepare the biomass modifier with good thermoplasticity; by using the biomass modifier for matrixasphalt modification of roads, low-temperature deformability of the matrix asphalt can be remarkably enhanced on the precondition of not changing high-temperature performance of the matrix asphalt.