The present invention provides siRNAs for inhibiting the expression of plk1 gene, and the method for inhibiting the expression of plk1 gene in mammalian cells. The siRNAs of the present invention have the double-stranded structure, and said double-stranded structure is composed of the first single strand and the second single strand that are fully complementary, wherein the sequence of said first single strand is the same as the target sequence within the sequence as shown in SEQ ID NO: 1, and the sequence of said second single strand is complementary to the target sequence within the sequence as shown in SEQ ID NO: 1. The siRNAs of the present invention can sequence specifically mediate the inhibition of plk1 gene expression, and have a good serum stability. By the introduction of the siRNAs of the present invention into the tumor cells, the expression of plk1 gene can be effectively inhibited, and the growth of tumor cells is inhibited and the apoptosis of tumor cells is promoted.