During microbial-induced calcium carbonate precipitation (MICP), the cementation medium needs to fully contact and react with bacteria inside a soil sample through pores of sand. The goal of this study was to develop a new sample preparation mold, i.e., a full-contact flexible mold, to prepare samples during the MICP process. The mold was constructed using a needle-punched, nonwoven geotextile material. Specimens of different sizes were prepared using the full-contact flexible mold to evaluate the engineering soil properties improved by the MICP process. The results indicated that the mechanical improvement of the MICP-treated soil using the full-contact flexible mold is one order of magnitude higher than that of the typical sample preparation method. The full-contact flexible mold enhances the contact between the bacteria and the cementation medium and promotes homogeneous calcium carbonate precipitation within the sand pores.