Abstract: Forty-millimeter by forty-millimeter by one-hundred-sixty-millimeter cement mortar specimens were fabricated and subjected to flow table tests, mechanical strength tests, and pre-crack repair performance evaluations. The influence of microbial self-healing agent content (0%−50%) on the workability, mechanical properties, and repair efficacy of cracks with widths of 0.2−0.4 mm and 0.4−0.6 mm was investigated. The correspondence between dosage and crack width was analyzed to optimize the repair efficiency, and the micromorphology of the deposited products within the crack zones was examined using scanning electron microscopy (SEM) to elucidate the underlying repair mechanism. The results indicate that the workability of mortar is significantly enhanced by the incorporation of the microbial self-healing agent, with a 50% dosage leading to a flow spread of 238.8 mm, representing a 64% increase compared to the reference group. However, the mechanical properties are observed to decline as the dosage rises, showing reductions of 36.2% in compressive strength and 62.0% in flexural strength at the 50% dosage level. The deposition products within the cracks are identified as calcium carbonate induced by microbial metabolism, whose crystal morphology is transformed from initial vaterite into stable calcite and densely accumulated in the cracks, thereby achieving efficient repair. Considering both the repair efficiency and strength retention, the dosage of the self-healing agent is recommended to be limited to within 20% to balance the repair effectiveness and mechanical performance. For cracks with widths of 0.2−0.4 mm, a dosage of 5%−15% is suggested, which can achieve a healing ratio exceeding 90% within 8−16 d, while for cracks of 0.4−0.6 mm, a dosage of 10%−20% is recommended, attaining a similar repair level within 16 days. This study provides appropriate dosage ranges of the microbial self-healing agent corresponding to different crack widths, offering reliable data support for its engineering applications.