The invention discloses a mushroom bran feed additive and a preparation method and application thereof. Through a four-stage fermentation technology, a laccase enzyme solution is directly added to mushroom bran, a fiber component is degraded by a microbial fermentation liquid, deep synergetic degradation is carried out on a fiber degradation compound enzyme solution, the flavor of a probiotic bacterial solution is regulated and controlled, and deep fermentation of the mushroom bran is progressively carried out in phases through synergetic degradation of microorganisms and a biological enzyme, thereby preparing the mushroom bran feed additive rich in dietary fiber, multiple essential amino-acids and microelements of iron, zinc and selenium. The mushroom bran feed additive is good in hydration property, can be added to feeds for livestock such as cattle, and is beneficial to growth and development of domestic animals; the quality of the domestic animals is improved. The preparation method is convenient and fast to operate; the problems that dismantled bags are accumulated to generate heat before the mushroom bran is fertilized, the temperature is difficult to control and flies and mosquitoes are easy to generate are solved; the mushroom bran feed additive has an application prospect in the aspect of industrialization of a mushroom bran feed.