A method of preparing a negative electrode active material of the present invention includes mixing a lithium precursor and a titanium precursor, and sintering the precursor mixture to prepare a lithium titanium-based active material including a lithium titanium oxide, wherein a residual amount of lithium in the lithium titanium-based active material is 2,000 ppm or less based on a total amount of the lithium titanium-based active material. The preparation method allows the residual amount of lithium to be 2,000 ppm or less in a range, in which rate capability is not significantly reduced, by appropriately controlling sintering temperature, wherein the method may provide a lithium secondary battery, in which an amount of gas generated is extremely small even if stored at high temperature, a thickness expansion rate is consequently considerably low, and, simultaneously, the rate capability is also excellent.