A structure of a storage box includes a main body, a stacking assembly, and a forward opening. The main body has an upper and front-facing opening and a recess on the rear. The stacking assembly is attached to the main body, covers the upper and front-facing opening, and has an upper cover and a front cover, the upper cover fitted to the front cover with a snap and having a convex portion, the shape of the convex portion identical to the shape of the recess. The forward opening is a part of the upper and front-facing opening of the main body and is covered by the front cover. When two or more storage boxes are stacked, the convex portion of the second storage box embeds the recess of the first storage box, preventing the entire structure from tilting forward when the front cover of the storage box above is opened.