The invention discloses a biomass fuel binder and a preparation method thereof. The binder is prepared from, modified lignin, hydrophobic acrylic resin, dodecyl alcohol ester, chlorosulfonated polyethylene, silicone defoaming agent, a curing agent and a plasticizer; the method comprises the steps of 1) mixing the hydrophobic acrylic resin and the dodecyl alcohol ester, slowly adding the modified lignin and stirring to prepare an emulsion A; 2) adding the chlorosulfonated polyethylene in the emulsion A in 1), adding the silicone defoaming agent, the curing agent and the plasticizer during the stirring and mixing process, and after uniformly stirring, drying, crushing and sieving to prepare the biomass fuel binder. The biomass fuel binder has the advantages of simple preparation process, strong cohesiveness and good water repellency, can effectively improve the molding rate, durability and moisture resistance of a biomass fuel, and is beneficial to long-term storage of the biomass fuel.