The strength ratio between two blanks of tailor welded blanks (TWBs) plays an important role in the formability aspect. To study the strength ratio effects on the formability of TWB is significant while more and more TWBs were applied in auto industry. The formability of TWBs square cup was analyzed by numeric simulation technology in the terms of different strength combination TWBs. The results indicate that the formality goes worse dramatically when the strength ratio bigger than 2.0 and the crack located on the higher strength blank side when the crack appeared. The results are applicable.