The invention discloses a method for directly synthesizing basic magnesium sulfate whiskers by brine. The brine after salt manufacturing, ammonia water and magnesium sulfate heptahydrate are taken as raw materials to synthesize the basic magnesium sulfate whiskers. The method comprises the following steps of: performing thermostatic hydrothermal reaction on the pretreated brine, ammonia water andmagnesium sulfate heptahydrate in a high-pressure reaction kettle to prepare basic magnesium sulfate whisker suspension, wherein the molar ratio of magnesium ions (Mg2+) of the brine to the ammonia water (NH3.H2O) to magnesium sulfate (MgSO4.7H2O) in the reaction kettle is 1.0:2.0-2.2:2.0-3.0, the stirring speed is 300 to 500rpm, the temperature is raised to 120-200DEG C, and the reaction time is4 to 8 hours; and performing solid-liquid separation on the basic magnesium sulfate whisker suspension, washing the solid with running water, and drying a filter cake obtained after washing to obtainthe basic magnesium sulfate whisker product. The technological process is simple, the cost of raw materials is low, the energy consumption is low, the product purity and quality are high, and after the synthesized whiskers are added into a composite material taking rubber and plastics as a matrix, the composite material has good reinforced property and flame retardancy.