In this paper, we present an analysis of, to the best of our knowledge, a novel class of the thin multi-trench-assisted optical coupler for multi-sensing of chemical detection. The design structure is an optical waveguide coupler consisting of closely coupled trenches filled with different liquids for chemical sensing. The refractive index profile of the coupler is allowed to vary due to the chemical filled in these trenches. The scalar finite difference method (FDM) is used to analyze the TE (s polarized) as well as TM (p polarized) modes, and thereafter propagate them along the structure through various trenched sections. We excite the structure with the eigenmode profile, which can be well approximated by the Gaussian shape pulse. The trenches with different chemicals and separation between them are well demonstrated in several examples. The main finding of this paper is that the coupling period of the structure can be changed by altering the refractive index of the trench region. In the FDM analysis, we have used the variable discretization step for better accuracy of these results. A new approach based on a thin multi-trench-assisted optical coupler scheme to analyze and design the chemical sensor is then proposed for the first time, to the best of our knowledge, for the detection of adulteration present in, for example, beverages, and in the medical industry. © 2018 Optical Society of America