The invention which belongs to the technical field of lithium ion batteries especially relates to a lithium ion battery and a negative electrode active material thereof. The negative electrode active material comprises graphite, silicon nanometer wires growing on graphite, and an amorphous carbon layer coated on the silicon nanometer wires. The lithium ion battery prepared with the negative electrode active material has the characteristics of high first efficiency, high capacity, low expansion, excellent cycle performance and the like. The invention also provides the lithium ion battery with the negative electrode active material.