Abstract: To elucidate the influence mechanism of carbonation treatment on the cementitious properties of stainless steel slag under low carbonation efficiency, this study systematically investigates the evolution of mechanical properties and microstructure under different carbonation efficiencies (CE). A series of samples with CE ranging from 1.59% to 12.06% were prepared by controlling carbonation reaction time. The phase composition, pore structure, and micro-morphology/element distribution of the interfacial transition zone (ITZ) were characterized using thermogravimetric analysis, X-ray diffraction, mercury intrusion porosimetry, and scanning electron microscopy with energy-dispersive spectroscopy. The results reveal that the cementitious strength exhibits a distinct “U-shaped” variation with CE, with two critical thresholds: a strength minimum at approximately 3.71% CE and a performance compensation point at about 4.33% CE. When CE is below 3.71%, the carbonation product CaCO3 forms an “isolated encapsulation” layer on the slag particle surface, which hinders further hydration and introduces harmful micron-sized pores, leading to strength deterioration. Once CE exceeds the threshold, the carbonation reaction penetrates into the C2S phase, and the generated CaCO3 transforms into a “continuous grid-like” structure. This transformation significantly refines the matrix pores through a micro-aggregate filling effect and promotes the dense precipitation of C-S-H gel by providing abundant heterogeneous nucleation sites. Analysis of ITZ indicates that deep carbonation narrows the ITZ width from 20–30 μm to less than 6 μm, and its internal structure transitions from porous and loose to a homogeneous, dense layer interwoven with CaCO3 and C-S-H gel. This study clarifies the comprehensive mechanism by which the degree of carbonation, under low CE conditions, dominates the U-shaped evolution of macroscopic properties through regulating product morphology and distribution, thereby reshaping the pore structure and ITZ. The findings provide a theoretical basis for the low-carbon and high-efficiency resource utilization of stainless steel slag.