Automatic summarization is need of the era. Mathematical method can help logic analysis. However, there is no any previous study about mathematic description of automatic summarization. A mathematic model of automatic summarization is established and discussed in the paper. The model makes use of meta-knowledge to describe the composition of summary and help to calculate the semantic distance between summary and source document. It is proposed that how to get meta-knowledge aggregate and their weight are the key problems in the model. There is also a discussion of how to select output summary in the paper. The mathematic description of automatic summarization is useful for the application of automatic summarization and automatic assessment for summaries. An experiment and results are described in the paper to explain the application of mathematic model in details.