A sandwich immunoassay theoretically exhibits higher sensitivity and specificity compared to a competitive counterpart; however, it is extremely difficult to obtain a pair of antibodies that can bind to a small molecule simultaneously, which is always thought to be a single epitope. In the present study, abamectin (ABM) was selected to prove the effect of hapten design and antibody recognition properties on the development of a sandwich immunoassay for small molecules. First, the epitopes of ABM were roughly located, and epitope distances were determined. Then, two haptens were designed by introducing spacer arms at the C4″-OH and C5-OH of ABM, respectively, aiming to provide the longest epitope distances. A total of seven rabbit polyclonal antibodies (pAbs) and 21 mouse monoclonal antibodies (mAbs) with various recognition properties were obtained. Extensive combinatorial associations of antibody pairs for simultaneously binding to ABM were performed, and only two mAb-mAb pairs were observed to achieve a sandwich immunoassay for ABM with a total success rate of 0.27%. The best mAb pair for sandwich immunoassay was confirmed by surface plasmon resonance, used to develop a sandwich immunoassay, and then evaluated by cross-reactivities and molecular docking with structurally similar analogues and abamectin. Altogether, the study provided a theoretical foundation as well as practical experience and demonstrated the importance of careful hapten design and extensive antibody screening to successfully establish the sandwich immunoassay for small molecules.