Broken formations are often encountered during drilling, which is prone to the instability of wellbore wall and induces serious downhole accidents. Enhancing the cementing force between the broken blocks and improving the mechanical strength of the surrounding rocks of a wellbore is one of the effective ways to solve the instability of wellbore in this type of formation, for which the microbially induced calcite precipitation (MICP) technology provides a good reference. To this end, the growth of Bacillus pasteurii in the solid-free drilling fluid system was analyzed through experimental study, and thus a solid-free drilling fluid suitable for its growth was obtained. Ultimately, a microbial solid-free drilling fluid system was constructed. Then, the effect of wellbore wall enhancement on microbial solid-free drilling fluid system and its mechanism were discussed using a core immersion experiment and scanning electron microscope (SEM) analysis methods. The results show that the most suitable environment for the growth of Bacillus pasteurii is the solid-free drilling fluid composed of plant gel, PAM, CMC, and culture medium, and the OD600 reaches 1.54 after 24 hours of growth. Therefore, it is determined that the formula of the microbial solid-free drilling fluid system is: 0.1% plant gel + 0.1% PAM + 0.1% CMC + Bacillus pasteuri (OD600=0.8) + 0.25% sodium chloride + 1% urea + 0.75% casein peptone + 0.25% soy peptone. Adding calcium sources to microbial solid-free drilling fluid can achieve a better effect of wellbore wall enhancement, and the compression strength of the consolidated specimen can be up to 0.183 MPa. Generally, the wellbore wall enhancement mechanism of this system is to generate calcium carbonate crystals under the induction of microorganisms in the loose particles and thereby fill in the pores, which enhances the cementation force between the loose particles so that the originally loose gravel soil could form an entirety, thereby enhancing the wellbore wall.