A method for synthesizing lithium titanium oxide-based anode active material nanoparticles, and more particularly, a method for synthesizing lithium titanium oxide-based anode active material nanoparticles using a supercritical fluid condition is disclosed herein. The method may include (a) preparing a lithium precursor solution and a titanium precursor solution, (b) forming lithium titanium oxide-based anode active material nanoparticles by introducing the lithium precursor solution and titanium precursor solution into an reactor at a supercritical fluid condition, and (c) cleaning and drying the nanoparticles, and may further include (d) calcinating the nanoparticles at 500-1000° C. for 10 minutes to 24 hours after the step (c).