The utility model relates to an expansion module structure of a programmable controller. The expansion module structure of the programmable controller comprises an upper shell and a lower shell, and is characterized in that upper terminal blocks are arranged at the front and back ends of the upper shell respectively, dustproof covers are arranged at the front and back ends of the lower shell respectively, and each upper terminal block comprises 7 wiring terminals. The number of the input/output points of an expansion module of the utility model is 8, and the upper and lower shells of the expansion module structure also can be used as an expansion module of which the number of the input/output points is 16, so that two sets of original and respective injection molds are combined into one set, and the number of the injection molds is reduced by 50%, and accordingly, the manufacture cost is saved. The upper and lower shells of the expansion module of the utility model can be suitable for the expansion module of which the number of the input/output points is 8 or 16, so that the product types are reduced by 50% during the production process, and the production efficiency and the product assembly quality are improved, at the same time, the order and storage costs of a company are reduced substantially, and the product management cost is reduced effectively.