The invention relates to water treatment and application, in particular to an adsorbent for removing heavy metal in water and a preparation method thereof. The adsorbent takes titanium salt and manganese salt as raw materials, the pH value of the raw materials is adjusted to 4 to 10, and the ageing is performed, so as to obtain the adsorbent, wherein the molar ratio of titanium to manganese is (9:1)-(1:3). Titanium and manganese composite oxide prepared by the method is aggregate of nano-particles, has a larger specific surface area, is high in adsorption rate, high in adsorption capacity, and environment-friendly, and can be used for removing heavy metal ion pollutants, such as lead, cadmium, arsenic, chrome, copper, nickel, cobalt, zinc and the like in various kinds of water, such as underground water, surface water and the like.