The invention belongs to the field of preparation of inorganic chemical materials, and particularly relates to a novel method for synthesizing basic magnesium sulfate whiskers at low cost. The methodcomprises the steps that concentrated seawater after sea water desalination and an inorganic alkali liquid are taken as raw materials, firstly a magnesium hydrate suspension liquid is prepared at thetemperature condition of 10-90 DEG C under a certain stirring and feeding rate, filtration and washing are carried out, a solid phase is dispersed into a sulfate solution, a hydrothermal reaction is carried out for 2-24 hours at 10-250 DEG C, then through filtration, washing and drying, the hydrated basic magnesium sulfate whiskers with uniform sizes, uniform shapes and better dispersibility are obtained, the composition of the hydrated basic magnesium sulfate whiskers is 5Mg(OH)2 MgSO4 3H2O, the average length of the hydrated basic magnesium sulfate whiskers is 40-400 microns, and the averagediameter of the hydrated basic magnesium sulfate whiskers is 0.5-4.5 microns. The novel method for synthesizing the basic magnesium sulfate whiskers at the low cost has the advantages that the process is simple, the cost is low, the added value is high, the temperature operation is flexible, and the operation and popularization are easy; a new way is provided for the high value utilization of concentrated seawater and salt lake magnesium resources.