Belonging to the field of biotechnology, the invention discloses a non-small cell lung cancer (NSCLC) marker, which is STAT3, and also can include CEA, CA125 and CYFRA21-1. The invention confirms the high expression of STAT3 in peripheral blood and serum, and discloses application of STAT3 in preparation of NSCLC diagnostic reagents by the inventor. Specifically, by making use of a genetic recombination technology and targeting at the gene coding region of STAT3, an eukaryotic expression vector PSUPER-STAT3 able to transcribe in vivo and generate small interference RNA (siRNA) is constructed successfully, and is transfected into an eukaryotic cell, thus laying a foundation for a further experimental study on lung cancer RNAi (RNA interference) and antitumor gene therapy. Utilization of the RNAi technology can effectively inhibit the gene expression of STAT3, induce cell apoptosis, and establish the base for molecular mechanism research and gene therapy of lung cancer, thus providing application of the marker provided in the invention in preparing related targeted medicines.