The invention relates to a preparation method of low-carbon Q and P steel for a 1400 MPa grade bainite/martensite multiphase vehicle. The low-carbon Q and P steel is prepared from the following chemical components including C, Si, Mn, Ni, Ti, Nb, Cr, Al, V, P, N, S, B and the like. According to a given composition formula of the invention, the preparation method comprises the following steps of after burdening, performing smelt with a converter, then performing secondary refining with a vacuum furnace, adding trace alloy elements, performing continuous casting to obtain a casting billet, performing heating and thermal insulation, removing phosphorus with high-pressure water, performing hot rolling, and performing water quenching to a room temperature; performing pickling and multi-pass cold rolling to acquire a steel plate with a target thickness; and heating the steel plate to A3 or above for thermal insulation, performing water quenching to T0 between MS and Mf for thermal insulation, and then performing air cooling to room temperature. In the preparation method, by adding various trace alloy elements, the contents of commonly used C, Si and Mn of the traditional steel are reduced, the welding performance and the formation performance are improved, and crystal grains are fined; and by regulating a thermal treatment technology to acquire mixed tissues of bainite, martensite and austenite, and good ductility and toughness are acquired while the strength is improved, so that the steel plate has good comprehensive mechanical performance and is beneficial to light weight, energy conservation and emission reduction of the vehicle.