1. A corpus expansion system, comprising: a corpus collection unit, for collecting corpus using sample seeds; a sample seed expansion unit, for generating new sample seeds, based on the sample seeds having been used by the corpus collection unit and the collected corpora; a balancing unit, for determining a corpus expansion strategy based on all the sample seeds having been used by the corpus collection unit and the new sample seeds; and a refining unit, for refining the new sample seeds based on the corpus expansion strategy; wherein said corpus expansion strategy includes at least one of the information about the class in specific field in which the sample seeds need to be expanded and the information about which sample seed needs to be deleted in said new sample seeds, wherein said refining unit refines the new sample seeds based on at least one of the information about the class in specific field in which the sample seeds need to be expanded and the information about which sample seed needs to be deleted in said new sample seeds.