A small interfering RNA for gene knockdown of the N-methyl-D-aspartate receptor NR1 subunit comprises 21 to 25 ribonucleic acids, which are homologous to the RNA sequence of N-methyl-D-aspartate receptor NR1 subunit. A method of using the small interfering RNA, applying the small interfering RNA on subcutaneous tissues temporary interfere with the genetic expression of the NMDA receptor NR1 subunit in hypoderm. A use of the small interfering RNA on pharmaceutics, applying the small interfering RNA manufacture into new analgesic drugs for moderating the inflammatory pain or intolerable chronic pain, especially on clinical chronic pain and burn pain patients. An analgesic drug for skin inflammatory pain comprising: the small interfering RNA and a siRNA acceptable vehicle.