The invention discloses high-security lithium ion batteries and a preparation method thereof. A prepared negative pole piece is coated with a chemically inert organic polymer coating, the organic polymer coating is insoluble in water, and the melting point of the organic polymer coating is 90-110 DEG C. when the lithium ion batteries have thermal runaway of battery internal short circuit caused by destructive crash such as overcharge, heavy stuff impact, thermal shock, short circuit, needling, and extrusion, a polyethylene nanoparticle coating is melted and forms a coating of the surface of the negative pole piece, a protective film is formed in the surface of the negative pore piece, the contact short circuit of a positive pore piece and the negative pore piece can be effectively isolated, the inlay and precipitation of lithium metal ions can be effectively prevented, and therefore the further rise of the temperature due to active substance dissolution and the thermal runaway is restrained, and the overall security property of a cell can be improved obviously.