The invention relates to a water purifying agent made of shells and a using method thereof and is characterized in that: the shells like conch shells, abalone shells, clam shells, oyster shells and coral shells are taken as raw materials, or any one of the shells is selected, and the shells are put in a heating furnace and is directly or indirectly heated by oxygen and natural gas, or electricityor an electric impactor; the heated shells are cooled and then prepared into strong basic calcium powder with a pH value of 10-15 by a pulverizing machine or pulverizing process, and the pores of thegranule bodies of the calcium powder are internally adsorbed with oxygen. The shells are heated at high temperature of 1,000 DEG C-3,000 DEG C for 30 seconds-30 minutes to be transformed into strong basic calcium matter. The shells are pulverized and processed into 100 meshes to 3,000 meshes powder to obtain the water purifying agent. The water purifying agent of the invention can modify water toalkalinity to prevent chlorella in fresh water from growing, can enrich the oxygen in fresh water to better purify water and kill harmful bacteria in fresh water and can repel and kill jelly fish to purify water.