The invention provides a lysine preparing method which ferments the lysine through the product of dry sugar manufacturing, divides the fermenting process into four steps, namely, increasing step of the number of lysine-producing microbe, first decreasing step of the number of lysine-producing microbe, retaining step of the number of lysine-producing microbe and second decreasing step of the number of lysine-producing microbe, adopts respective ventilation control, so as to enable the lysine-producing microbe to better produce the lysine, improve the fermenting efficiency of the lysine, and reduce the production cost of the lysine. Furthermore, as the control of the fermentation process is adjusted, the product of the dry sugar manufacturing can effectively ferment, so as to highlight the low cost of the dry sugar manufacturing, and integrally improve the economy of the preparing technology of the lysine.