Factor space is an indispensable part of the model validation. In order to provide an advantageous method to build factor space for model validation, this paper states the challenging problems of the factor space, and proposes a mathematical model of it. Further based on the model, this paper provides the graphic illustration, the factor decomposition, the credibility aggregation and the model defect tracing of the factor space, which construct a novel method to build a factor space for model validation. Finally, the paper provides a case study of an electromagnetic rail gun model validation to explain the usage of the method.