The invention relates to a diselenide compound synthesis method and the adopted technical proposal comprises the following steps: adopting halogenated hydrocarbon and selenium as raw materials and organic base or inorganic base as cocatalyst or no cocatalyst, continuously injecting carbon dioxide gas in organic solvent to react in the present of water under the temperature of 20-100 DEG C at normal pressure for 1-24h, cooling to the room temperature, switching carbon dioxide to air or oxygen to precipitate unreacted selenium, filtrating to collect filtrate, then adding 2-3 times of water, precipitating products and finally obtaining the diselenide compound by filtration. The method invention is normal press reaction with low cost, environmentally friend, low reaction process difficulty, convenient operation and easy following separation of products and catalyst.