The key structural unit of iron-based superconductors (FeSCs) is the Fe2X2 (X refers to a pnictogen or a chalcogen element) layer which stacks alternately along the crystallographic c axis with other spacer layers. This structural feature makes it possible to find FeSCs via rational material design. In this paper, we first review the crystal structure of FeSCs along with the relevant progress. Then we summarize several rules for designing the intergrowth structures. The rules include the following points. 1) Lattice match between the intergrowth layers should be good enough. Quantitatively, the lattice mismatch, defined as =2(aA-aB)/(aA + aB), where aA and aB are respectively the lattice parameters of the two constituent compounds, should be no larger than~2%. 2) The charge transfer between the intergrowth layers is mostly essential, which acts as the glue that combines the constituent layers together. Such a charge transfer also induces the extra charge carriers in the superconducting key layer to give rise to superconductivity without extrinsic doping (so-called self doping). 3) For the structure with similar yet crystallographically distinct sites, one needs to avoid forming solid solutions. 4) Each intergrowth layer is preferably thermodynamically stable. 5) The designed structure can be preliminary evaluated with the hard and soft acids and bases conception and ab initio calculations. Following these empirical rules, we introduce and analyze five examples, namely, (Li0.8Fe0.2OH)FeSe, Ba2Ti2Fe4As4O, 42214-type Ln4Fe2As2Te1-xO4 (Ln=Pr, Sm, Gd), 1144-type AkAeFe4As4 (Ak=K, Rb, Cs; Ae=Ca, Sr, Eu), and 12442-type AkCa2Fe4As4F2 and AkLn2Fe4As4O2 (Ak=K, Rb, Cs; Ln=Nd-Ho). For the last 12442-type compounds, we also discuss the unusual relation between superconducting transition temperature and crystallographic parameters. We conclude that the structural-design approach may serve as an effective route, not only for discovering new FeSCs but also for exploring other relevant functional materials with similar crystal structures.