The invention discloses a method for preparing a lignin sulfonate dispersing agent by using a two-step oxidization method, which comprises the following steps: using pulping black liquid as a raw material; adding an acid regulator for regulating a pH value to be 10.6-13.5 at the temperature of between 30 DEG C and 40 DEG C; adding 1.0-5.3 percent by mass of oxidant solution to react for 20-90 minutes at the temperature of between 40 DEG C and 80 DEG C; adding 1.7-5.2 percent by mass of formaldehyde solution to react for 20-120 minutes at the temperature of between 60 DEG C and 100 DEG C; adding 2.0-4.6 percent by mass of sulfonating agent solution to react for 2-5 hours at the temperature of between 80 DEG C and 120 DEG C; reducing the temperature of the solution to be between 30 DEG C and 60 DEG C and adding the acid regulator for regulating a pH value to be 4-8; adding 2.5-10 percent by mass of oxidant solution to react for 1-6 hours at the temperature of between 30 DEG C and 60 DEG C; reducing the temperature and discharging; and spraying and drying to obtain the lignin sulfonate dispersing agent. The dispersing agent has the multiple functions of dispersing, complexing, chelating and the like, and can be applied to various fields of coal water slurry additives, dye dispersing agents, concrete water reducing agents, viscosity reducers for drilling fluid and the like; in addition, the preparation method is simple and has significant economic and social benefits.