The photo-induced phenomenon in the silicon nanowires made by chemical etching in HF/Fe(NO3)3 solution is investigated systematically by using monocrystal n-type silicons with different doping concentrations as substrates, silver as catalyst, and iron nitrates with different concentrations as oxidants. It is found that the length of silicon nanowires is determined not only by the doping concentration of substrate and the mass of oxidant, but also by the photo-induced effect. The prepared silicon nanowires may have potential applications in green energy storage device and the substrate material for sensor. In this paper, we discuss the formation mechanism from the band structure, electrochemical characterization and photoluminescence in depth. The results in this paper provide physical theoretical evidence for the development of the method, and have important guiding significance to promote the technology.