1. A method for expanding a seed document in a seed document set, wherein the seed document set comprises at least one seed document, the method comprising: identifying one or more entity words of the seed document, wherein the entity words are words indicating focused entities of the seed document; identifying, based on each identified entity word, one or more topic words related to the based entity word in the seed document where the entity word is located; forming an entity word-topic word pair from each identified topic word and the entity word as the basis for identifying the each identified topic word; and obtaining one or more expanded documents through the web by taking the entity word and topic word in each entity word-topic word pair as key words at the same time, wherein the expanded documents comprise not only the entity word in the each entity word-topic word pair but also the topic word in the each entity word-topic word pair.