Gene silencing is a kind of epigenetic phenomenon discovered in transgenic plants in recent years. Gene silencing can be divided into two types: position effect and homology dependent gene silencing (HdGS). Homology-dependent gene silencing (HdGS), which is the generic term for transcriptional gene silencing (TGS), post-transcriptional gene silencing (PTGS) and RNA-mediated virus-resistance (RmVR) have been shown to frequently occur in transgenic plants. Post-transcriptional gene silencing (PTGS) is an antiviral defense system of plants, which has led to a new understanding of the relationship between viruses and plants. And it's a co-evolution result between plants and viruses. A few hypothesis models have been proposed to elucidate the mechanism of gene silencing, but they could not explain all the phenomena in gene silencing. In this paper, we reviewed the relationship between PTGS and the plants defense mechanism to virus, and proposed a model of gene silencing based on our experiments results.