Hole wall instability in loose and fractured formations has always been one of the problems that plagued the drilling engineering community. Enhancing this type of formation and improving its mechanical properties are the technical keys to effectively solving the hole wall instability. In this paper， the microbially induced calcium carbonate deposition （MICP） technology is combined with the CMC solid-free drilling liquid phase to construct a microbial-CMC solid-free drilling fluid system. Through core immersion experiment， X-ray diffraction experiment （XRD）， and scanning electron microscopy， two microanalysis methods are used to explore the solid wall effect and microbial-CMC solid-free drilling fluid mechanism. The results show that the microbial-CMC solid-free drilling fluid has a more obvious reinforcement effect on loose and fractured formations. The longer the action time， the higher the initial bacterial concentration and the calcium source concentration， the better the wall-fixing effect. In the process of wall solidification， microorganisms penetrate the inside of the sample with the drilling fluid， induce calcium carbonate crystals between the loose particles， fill the pore space， cement the loose particles into a whole， and have a particular mechanical strength to achieve the reinforcement hole The purpose of the wall. The results of this study solve the problem of hole wall instability in loose and fractured formations and explore new drilling fluid technical solutions.