The invention relates to an activating agent for invalid-state iron in soil. The activating agent comprises the following components in parts by weight: 1-90 parts of master batch of the iron activating agent, 0-85 parts of filler, 1-5 parts of surfactant, 1-10 parts of anti-blocking agent and 0.1-5 parts of plant growth regulator. The activating agent can be used for directly activating the fixed microelement iron with rich content in the soil and further transforming the fixed iron to valid-state iron which can be absorbed and utilized by plants, thus reducing needed iron compounds and saving resources.