The present invention provides a complex molecule interfering the expression of target genes and the methods for preparing the complex molecule , wherein the complex molecule contains two siRNA strands X 1 and X 2 having at least 80% complementarity, the 5' end of X 1 and 3' end of X 2 are linked through non-nucleic acid molecule L 1 , the 5' end of X 2 and 3' end of X 1 are linked through non-nucleic acid molecule L 2 . Since both 5' and 3' ends of two siRNA strands X 1 and X 2 of the complex molecule according to the present invention are linked through non-nucleic acid molecules, it is not easy to unwind and degraded for the siRNA strands, and therefore the chemical stability of siRNA and the remaining time in the blood are greatly improved. After being administered, the Dicer enzyme in the cells is utilized to release the locked siRNAs from the complex molecules, and after unwinding, the antisense strand of the siRNA is released from the double-stranded siRNA to inhibit the expression of the target genes.
