<p>The bio-carbonation of reactive magnesia cement (RMC) method was coupled with the fiber reinforcement to enhance the mechanical properties of solidified construction and demolition waste (CDW) for geotechnical construction. The typical polypropylene fiber (PF) with different fiber content and lengths was employed to prepare the fiber-reinforced bio-carbonation of RMC solidified CDW (F-BC samples). Changes in the mechanical properties and the microstructures induced by fiber reinforcement were elaborated. Experimental results indicated that the fiber reinforcement presents an optimization method for enhancing the mechanical properties of F-BC samples, improving the UCS and toughness, reducing the significant brittleness, and mitigating instantaneous failure. With the higher fiber content, the UCS reached the maximum UCS of 2.41 MPa at 0.2% content. The residual strength increased monotonically, the brittleness index decreased, and the toughness significantly improved. With the longer PF, the UCS obtained the maximum value of 2.24 MPa at the 9 mm PF. The mechanical properties did not show significant changes with PF lengths. The included PF can be interwoven and anchored in the pores and surfaces of CDW by the bio-carbonation of RMC products to form 3D spatial network structures, restricting sample deformation. After sample failure, residual fibers across the failure surfaces can act as bridges to inhibit further deformation. Excessive fiber content and overlong fibers were detrimental to the reinforcement effect. The optimal PF content is 0.2%, and the optimal PF length is 9 mm. The results are of some significance for the geotechnical stability and safety.</p>