1. A method for merging document clusters, wherein a corresponding document cluster is generated for each descriptor, each document cluster comprises a plurality of documents, and document clusters having a high association degree are merged, the method comprising: establishing an oriented graph, further comprising: setting each document cluster as a node; making statistics on an amount of the same documents (document amount ) commonly presented in one node and another node; calculating an associated value ( documnet amount cluster_i _document total ) of the node according to an amount of documents in the node (cluster_i_doucment total ); and when the associated value of the node to another node reaches a threshold value, establishing an oriented edge to point from the node to the another node; determining whether nodes pointing to each other exist or not; if associated values ( documnet amount cluster_i _document total ) of the nodes pointing to each other are greater than a first threshold value, merging the nodes into an output node; if the nodes pointing to each other do not exist, finding out destination nodes having the same descriptor and nodes pointed to by the destination nodes; calculating an associated weight between the destination nodes and the nodes respectively; and if the associated weights of the destination nodes having the same descriptor are greater than a second threshold value, merging the nodes into an output node.