The invention relates to a lanthanide water treatment agent as well as a preparation method thereof, which is a novel water treatment agent. The chemical compositions of the agent are: a La2O3, b CeO2, c Pr6O11, d Nd2O3, e SiO2, f Al2O3, g MgO and a small quantity of unavoidable impurities, wherein, numeric areas of coefficients of a, b, c, d, e, f and g are respectively: 0.20<=a<=0.35, 0.32<=b<=0.55, 0<=c<=0.035, 0<=d<=0.035, 0.10<=e<=0.30, 0.03<=f<=0.09, 0.010<=g<=0.014. The water treatment agent can eliminate the organic pollution in water and can also eliminate most of inorganic pollution, and can treat general pollution and eliminate or inhibit the growth and reproduction of blue algae, green algae and red tide. The water treatment agent and the preparation method are favorable for treating the pollution of reservoirs, lakes, ponds and rivers with slow velocity of flow and for restoring the water ecological environment, and can also treat drinking water, culture water, irrigation water, landscape water, industrial wastewater, domestic wastewater, etc.