The invention discloses a normal-temperature and normal-pressure preparation method and application of a metal selenide mercury adsorbent. The metal selenide mercury adsorbent is prepared at normal temperature and normal pressure by adopting a precipitation method. The metal selenide mercury adsorbent comprises a compound formed by selenium and one or more of chromium, manganese, iron, copper, cobalt, nickel, zinc, molybdenum, silver and cadmium and a composite of the compound, unstable mercury is converted into stable mercury selenide, mercury in industrial tail gas or by-products of fire coal, metallurgy, waste incineration, cement production and the like is removed, mercury enriched in mercury-containing articles for daily use is absorbed, the recovery and reutilization of mercury resources are realized, and mercury to be treated can exist in the forms of a simple substance state, a free state and/or a compound state and the like. As the metal selenide has extremely high mercury adsorption capacity and adsorption rate, realization of large-scale preparation of the metal selenide under normal temperature and pressure conditions has great significance on mercury pollution prevention and control, and the corresponding technology has wide market application prospects.