Aiming at the small sample data of launch vehicle, combining the SPOT(sequential posterior odd test）and Bayes recursive estimation method to improve the traditional sample size design from hypothesis testing and parameter estimation, respectively. When evaluating the performance indexes of launch vehicle following the normal distribution, the composite equivalent coefficient was introduced to fuse multi-source data, which can make up for the shortage of experimental data or test data. Considering the two types of risks and confidence requirements comprehensively, a reasonable evaluation scheme was designed. Through the analysis of examples, the results verified that the proposed method are authentic and credible, which can reduce the sample size demand effectively, and can be used for the sample size test design of launch vehicle under the small sample condition.