The invention relates to a method for establishing and using failure criteria for a laser tailor-welded blank with different thicknesses. The method is mainly used for judging the necking or the fracture of the laser tailor-welded blank with different thicknesses and for estimating the forming performance. The method for establishing a forming limit diagram of the laser tailor-welded blank with different thicknesses comprises the following steps: the tailor-welded blank forming limit diagram is formed by combining a welding area material, and a forming limit curve of the thin tailor-welded lateral mother plate; and the forming limit curve of the tailor-welded material adopts the M-K theory and is obtained by calculating performance parameters by combining the tail-welded area material. The method for using the forming limit diagram of the laser tailor-welded blank with differential thicknesses comprises the following steps: the strain value of the danger part of the part of the tailor-welded blank is measured and is divided into the strains of the welding area and the thin lateral mother plate; the measured strain is compared with the welding area corresponding to the strain, and the forming limit curve of the thin tailor-welded lateral mother plate, and whether the part of the tailor-welded plate loses effectiveness is judged. The method for establishing and using the failurecriteria for the laser tailor-welded blank with different thicknesses accurately establishes the forming limit diagram, predicts successfully the failure of the tailor-welded plate, and effectively estimates the forming performance.