To study the impact of limestone powder (LP) fineness on the physical and mechanical characteristics of concrete, a total of 88 sets of LP concrete mixtures were designed, in which the LP content (0~35% of the mass of cementitious materials), LP-specific surface area (350–1000 m²/kg), and water–binder ratio were used as testing factors. The experimental and theoretical approaches were performed to study the physical (slump) and mechanical properties (compressive strength and bond-slip behaviors with steel bars) of LP concrete. Secondly, the compressive activity index (CAI) was introduced as a measurement for quantifying the compressive activity of LP. The results indicate an optimal improvement in the physical and mechanical properties of LP concrete when the LP content is equal to or less than 15% and the specific surface area is equal to or less than 600 m²/kg. An increase in LP content will result in a drop in the bond strength between concrete and steel bars; in contrast, an increase in LP-specific surface area will enhance the bond strength. Furthermore, CAI can better reflect the role of LP in concrete, which provides a theoretical basis for the application of LP in concrete.