The invention relates to a method for the primary purification of seawater. The method comprises the following steps: 1, crushing weathered coal to above 60 meshes; 2, mixing evenly one portion of 99.5 percent concentrated sulfuric acid and 10 portions of water, adding the diluted sulfuric acid into the weathered coal for mixing with the proportion of 1:1, and drying the mixture to reach the water content of 30 to 35 percent after 5 to 7 days of placement in a sealed status; 3, adding the dried weathered coal into seawater by the proportion of 1: 500, placing the seawater for 5 days at 10 to 60 DEG C, stirring twice every day for the first three days, and standing for the last two days; and 4, finally filtering out the precipitates in the seawater. The seawater processed by the method notonly can be directly used as industrial water, but also can be used as domestic water after being desalinated by a seawater desalination device. The method is simple and feasible, has low cost, needsno special equipment, and greatly reduces the probability of blocking a filter membrane during desalination.