A method for indexing a suffix tree in a social network includes: scanning an input string and dividing the string into partitions each having a common prefix; performing no-merge suffix tree indexing on the divided partitions; storing information on the partitions on which no-merge suffix tree indexing is performed; storing suffix nodes of the no-merge suffix tree; and establishing a prefix tree. The performing no-merge suffix tree indexing includes: generating a set of suffixes having the common prefix in the input string; generating a suffix set from the set of suffixes and storing the suffix set; and building the suffix set as a sub-tree.