Abstract:Plant activator (also called elicitor) itself and its metabolites have no fungicide activity in vitro, but they can induce plant immunity system to produce systemic acquired resistance (SAR) against a wide spectrum of pathogens persistently. Apart from accumulation of lignin in cell wall by increase of hydroxyproline rich glucoproteins (HRGP) induced by elicitor, increase of endogenesis salicylic acid (SA), oxidative burst, hypersensitive response (HR) caused by programmed local cell death could also be observed in the elicited plants. The persistent SAR against a wide spectrum of pathogens only occurred after the changes of circumstantial resistant enzymes and substances such as phenylalanine ammonialyase (PAL), β-1,3-glucanase, chitinase, peroxidase (POX), lignin and phytoalexins et al. caused by a series of regulation of plant guard gene expression through pathogenesis resistance signaling pathways conducted by endogenesis signaling compounds SA, jasmonic acid (JA), ethylene (Et) and nitrogen oxide (NO). In this paper, the conception, characteristics, category and mechanism of the mode of action of plant activator were discussed. The prospects and directions of application and development of plant activator were mentioned. The possible problems and the corresponding methods for settlement were also being put forward.