A fertilization amount regulating mechanism with fertilizer outlet area of a fertilizer box increasing or decreasing in arithmetic progression belongs to agricultural machinery; a fertilizer outlet area regulating plate is fitted to a lower side of a bottom plate of a granular fertilizer box and coordinates with a fertilizer outlet in the bottom plate, an inside profile surface of the fertilizer outlet area regulating plate is formed by serially connecting a linear segment A, an arc segment A, an arc segment B, an arc segment C, an arc segment D, an arc segment E and an arc segment F, a fertilizer outlet profile surface is formed by connecting an arc segment G, a linear segment D, a linear segment E, an arc segment H, a linear segment F, a linear segment B and a linear segment C in an enclosed manner; the fertilizer outlet area of the bottom plate of the granular fertilizer box is increased or decreased in arithmetic progression through the fertilizer outlet area regulating plate so that fertilizer discharge amount is under control; the mechanism has the advantages of novel structural design, reasonability, simplicity, regulation convenience, and high precision.