A water treatment agent is prepared from, by mass, 52.0-61.0% (W/W) of sodium chlorite, 7.5-7.9% (W/W) of sodium bicarbonate, 7.1-7.5% (W/W) of sodium carbonate, 0.3-1.2% (W/W) of calcium chloride, 5.0-18.0% (W/W) of sepiolite and the balance sodium chloride. A method for preparing the water treatment agent comprises the steps of 1, weighing sodium chlorite powder, sodium bicarbonate powder, sodium carbonate powder, sodium chloride powder and sepiolite powder, placing all the powder in a reaction still, increasing temperature to 75 DEG C while stirring at the heating rate of 12 DEG C per min, and then conducting mixing for 28-35 min; 2, reducing temperature to 35 DEG C, then adding calcium chloride powder, and conducting mixing for 28-35 min; 3, directly conducting powder packaging according to packaging requirements, or conducting granulation in a granulator and then conducting packaging. Compared with the prior art, the water treatment agent has the advantages that oxidizing property is high, a broad-spectrum sterilization effect is realized, all bacterial reproductive bodies and pathogenic bacteria can be killed under the condition of 0.1 ppm, bacterial reproductive bodies can be completely killed under the condition of 50 ppm, and using is convenient.