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 in memory by a processor, wherein the one or more identified entity words are words indicating focused entities of the seed document; identifying by the processor, based on each of the one or more identified entity words of the seed document, one or more topic words related to each of the one or more identified entity words, the one or more identified topic words located in the seed document; forming, by the processor, an entity word-topic word pair from each of the one or more identified topic words and each of the one or more identified entity words upon which each of the one or more identified topic words is identified; and obtaining one or more expanded documents by the processor by taking the entity word and topic word in each entity word-topic word pair as key words for web searching 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.