The pursuit of the limit between dimensionalities is a scientific goal with high applicability. Sandwich immunoassay, usually based on two antibodies binding two epitopes, is one of the most popular mainstay tools in both academic and industrial fields. Herein, we determined and evaluated the minimum distance of two epitopes in sandwich immunoassays for small molecules. Briefly, nine model analytes comprising two hapten epitopes, i.e., melamine (MEL) and p-nitroaniline (NIA), were designed by increasing the linear chain linkers brick by brick. Two groups of monoclonal antibodies (mAbs) were produced with different recognition properties toward MEL and NIA using 12 new haptens with different spacer arms. The results indicated that two epitopes of the analyte with a distance of only 2.4 Å could be simultaneously bound by two mAbs, which is the known limit of epitope distance in sandwich immunoassays thus far. We further found that an epitope distance of below 8.8 Å for the analyte generally induces noticeable steric hindrance of antibodies, preventing a sandwich immunoassay with high probability. These observations were investigated and evaluated by molecular docking, molecular dynamics, and surface plasmon resonance and using model and real analytes. Altogether, we determined the minimum distance of two epitopes and explored the molecular mechanism of the antibody–analyte–antibody ternary complex in sandwich immunoassays, providing a theoretical basis for hapten design, antibody discovery and development, and sandwich immunoassay establishment for small molecules.