The invention discloses a processing method of fine bastnaesite. The processing method comprises the following steps: (1) carrying out ball-milling treatment on fine bastnaesite, so as to obtain fine bastnaesite particles; (2) mixing the fine bastnaesite particles with concentrated sulfuric acid, and curing, so as to obtain cured ores; (3) roasting the cured ores, so as to obtain roasted ores; (4) carrying out slurry mixing, water leaching and filtration on the roasted ores, so as to obtain leachate and leached slag; (5) extracting the leachate, so as to obtain rare earth and thorium-containing extraction liquid; (6) carrying out back extraction on the thorium-containing extraction liquid, so as to obtain thorium nitrate. By virtue of the processing method, Sichuan fine bastnaesite can be effectively processed, furthermore, the flow is simple, the REO leaching rate is high, the energy consumption is low, the consumption of auxiliary materials is low, meanwhile, the content of radioactive thorium is greatly decreased into a standard range, fluorine and radioactive thorium are not emitted in the whole process, the leached slag can be directly emitted after being washed, the environment is protected, furthermore, rare earth can be effectively separated, and fluorine and thorium resources can be comprehensively utilized, so that the processing method is a novel clean production process for metallurgical treatment of fine bastnaesite.