The invention discloses a papermaking wastewater treatment agent and an application method thereof. A wastewater treatment agent is prepared from calcium oxide, sodium carbonate, aluminum polychloride, polyferric chloride, magnesium sulfate, seaweed soil, corn starch, activated carbon, sodium acetate, sodium chlorite and sodium bicarbonate. The application method comprises the following steps: in wastewater, adding the calcium oxide and the sodium carbonate, and then adding the aluminum polychloride, the polyferric chloride, the magnesium sulfate and the seaweed soil; removing precipitate and impurities, then adding the activated carbon and the sodium acetate, and finally adding the sodium chlorite and the sodium bicarbonate for sterilization and disinfection. The papermaking wastewater treatment agent has dual functions of flocculent precipitate purification and sterilization and disinfection for wastewater, therefore can remove sediment and impurities in the wastewater, can also remove harmful substances, and carries out sterilization and disinfection. The wastewater treatment agent has the advantages of being wide in raw material source, low in cost, good in treatment effect and high in efficiency, cannot produce secondary pollution, saves energy, protects environment, and is convenient for popularization and application.