In order to detect the expression of messenger ribonucleic acid (mRNA) of the breast cancer susceptibility gene 1 (BRCA1) ) in real time in living cells, a novel gold-nanoparticle-based fluorescent molecular beacon is designed and successfully prepared. The specificity, sensitivity, stability and toxicity of this beacon are investigated, the fluorescence recovery strength of this beacon is detected by the techniques of laser confocal microscopy and flow cytometry, and thus the expression of BRCA1 mRNA in human gastric cancer cell BGC823 and its drug-resistant cell BGC823/DDP is realized. The reverse transcription polymerase chain reaction （RT-PCR） technique is used to verify the reliability of this beacon for detecting the expression of BRCA1 mRNA in cells. The results show that this beacon has excellent characteristics and can be reliably used to detect the expression level of BRCA1 mRNA in tumor cells.