The invention relates to an alterant of an iron-rich phase in secondary aluminum and an alteration method. The alterant is composed of a [Mn] agent and a [B] agent. The alteration method includes the steps that part of secondary aluminum is heated to form a melt, then the [Mn] agent is added, the remaining secondary aluminum is added after the [Mn] agent melts, the [B] agent is added, refining is carried out after the [B] agent melts, pouring is carried out after standing is carried out for a period of time, and the secondary aluminum obtained after alteration treatment is obtained. According to the alterant and the alteration method, the Fe element in the iron-rich phase can be replaced through the [Mn] agent, the advantage growth orientation of the iron-rich phase is changed, and therefore a needle-like beta-Fe phase is eliminated; meanwhile, the forming temperature of the iron-rich phase can be reduced through B in the [B] agent, the growth time of a primary iron-rich phase is shortened, the growth space of the primary iron-rich phase is reduced, the B can also serve as a surface-active element, and is absorbed to the surface of the iron-rich phase in the initial phase of formation of the iron-rich phase, and growth of the iron-rich phase is restrained, so that through the combined action of the [Mn] agent and the [B] agent, existence of the needle-like iron-rich phase and the primary iron-rich phase can be completely eliminated, the uniform Chinese character type iron-rich phase is obtained; and in addition, the adding amount of the Mn can be greatly reduced, and the mechanical performance and the machining performance of the secondary aluminum can be greatly improved.