In split Hopkinson tensile bar test, it is necessary to obtain the smallest strain rate of the specimen which was fractured by a tensile stress impulse, while determining the corresponding launch pressure of the smallest strain rate needs to conduct many tentative tests. In order to solve this problem well, an empirical method which needs to carry out tensile fracture test only once was proposed to calculate the smallest launch pressure and strain rate to fracture the specimen. The method can further predict the strain rate in different launch pressure. The tensile fracture tests of four kinds of aluminum alloy testify that the proposed method is coincident with the experimental results and it is efficient to evaluate the launch pressure and strain rate.