The invention discloses a method for preparing a bactericide N-(2,4,6-trichlorophenyl) maleimide. The method comprises the following steps of: firstly synthesizing an intermediate product of N-(2,4,6-trichlorophenyl) maleimide at normal temperature; and then carrying out catalytic dehydration on the intermediate product to obtain the N-(2,4,6-trichlorophenyl) maleimide. The method has the advantages of low reaction temperature, low industrialization cost, little harm and high yield up to 80%.