The overall test of the complex and high-cost system is small sample in general. Therefore, some prior information is needed for the evaluation of the complex system, and results in the information of the overall small sample test submerged. Different from the general weight method, one type of Bayes formula is deduced based on the normal inverse Gamma distribution of accuracy evaluation, the prior information is equivalent to one overall test data and the corresponding distribution parameters are designed, and the posterior result is deduced by combining the prior sample. Finally an example is provided to analyze the formula's validity and its application area.