Disclosed are an index structure and a method of extending index which comprises: (a) performing indexing operations of generating inverted index for newly inserted data source in the memory; (b) if the number of source data involved in the indexing operations reaches a first threshold value k1, sequentially writing the generated inverted index into the first index subfile; (c) if the number of the smallest grids, or index groups, in the first index subfile reaches a second threshold value k2, merging the k2 grids into a larger grid and sequentially writing it into the second index subfile; and (d) if the number of the smallest grids in the second index subfile reaches a third threshold value k3, merging the k3 grids into a larger grid and sequentially writing it into the first index subfile. Because index updating mostly occurs in small grids, the number of I/O operations on large grids is reduced and thus the speed of index building and updating is increased. In addition, the threshold values k1, k2 and k3 may be automatically adjusted based on the usage of system resources.