The invention discloses nucleic acid and a kit used for detecting polymorphism of A118G locus of OPRM1 gene of human beings, and establishes a method for detecting polymorphism of A118G locus of OPRM1 gene of human beings with strong specificity, high sensitivity, high accuracy and simple operation, and provides a guide scheme for individual dose administration of opioid, such as morphine, fentanyl, tramadol, oxycodone & aceta minophen and dolantin. The detection method provided in the invention adopts an operation in a completely-closed tube, the operation is simple, convenient and rapid, the detection result can be directly obtained by detecting the fluorescent signal value in the PCR process, and no PCR post-treatment or electrophoresis detection is needed, thereby overcoming the difficult problems of easy pollution of the traditional PCR technology, easy false positive rate, and capability of effectively avoiding non-specific amplification, and being applicable to detection of large-scale samples.