Powertrain mounting bracket is a vital part in the car, its strength is more important for the vehicle lifetime, and lightweight is becoming the aim of structure design. According to the optimization design idea, a common formula of size optimization was deduced. And the structure of the mounting bracket was analyzed by using Optistruct software and the bracket in real work condition was studied. In the premise of meeting the stress, the optimal design of the bracket was finished. And the result show that the optimal design is right and reasonable, the quality of optimized bracket was reduced 37.28%, and achieved good results.