The invention discloses an agricultural magnesium sulfate fertilizer and a preparation method thereof; wherein, the preparation method thereof comprises the following steps: (1) magnesia ore powder and water are fully stirred in a slurry mixer and prepared into magnesia slurry; (2) the mixed magnesia slurry is fed into a mixed reactor to be mixed and reacted with the vitriol continuously added into the mixed reactor; the adding amount of vitriol is adjusted to lead the reaction temperature of the mixed slurry at the outlet of the mixed reactor to be controlled between 121 DEG C and 155 DEG C;(3) the reacted mixed slurry is fed into a returning formation chamber for further reaction and curing, and the water in the materials is baked and vaporized by the self reaction heat of the mixed slurry to dry and solidify the materials; and (4) the cured mixed materials are shaped and packaged, thus becoming the finished product of the agricultural magnesium sulfate fertilizer. The preparation method of the agricultural magnesium sulfate fertilizer has the advantages of high production efficiency, high degree of mechanization, no waste water and solid waste discharge, and small pollution.