Objective：To construct the siRNA expression vector which targeted the human Kir 4.1 gene，to study the impact of the Kir 4.1 on the proliferation of U-251 by RNAi and to explore the new method in astrocytoma treatment. Methods：Cell culturing，transforming，flow cytometry screening，RT-PCR and Western blot were used to detect recombinant plasmid and investigate the cell cycle change（G1 phase，S phase and G2 phase） of U-251 cell in control group，negative control group and transformed cells. Results：Three interference vectors were constructed successfully. A plasmid which didn’t induce Kir 4.1 mRNA production was obtained. Flow cytometry screening showed that the cell number in each phase of negative control group had no apparent difference while in the transformed group the cell number in G2 phase decreased from 19.39% to 1.5%（P=0.018），the cell number in S phase increased obvi－ously from 36.32% to 51.22%（P=0.027）. Conclusion：The siRNA expression vector which targeted the human Kir 4.1 is constructed successfully. Kir 4.1 gene is silenced in the transformed U-251 cells by siRNA expression vector. U-251 cell stops at S phase proba－bly by inhibiting the G2 phase，which can decrease cell proliferation.