The microstructure of the interfacial transition zone (ITZ) between cement paste and aggregate influences the properties of concrete. Meanwhile it depends strongly on the nature of the aggregate, specifically its porosity and water absorption. In this study, a porous and low carbon material biochar which is recycle used from bamboo biomass is incorporated as part of the fine aggregate (BFA) with different contents (5–20%) in the HPC. The ITZ associated the HPC’s mechanic properties, including compressive strength, flexural strength, and the bonding strength by pullout test of reinforcement are investigated. The internal temperature and also relative humidity of BFA-HPC are monitored and evaluated. Backscattered electron imaging examinations is adopted to investigate and analyze the underlying mechanisms of BFA’s internal curing performance and ITZ reinforcement in HPC. Combined with the tracking test on the hydration process and microstructure evolution of HPC, the multifaceted regulation mechanism of the concrete microstructure enhancement by internal curing mechanism and the tank effect within the BFA is revealed. However, the natural pores of BFA impaired the compressive strength, the bonding strength and ITZ of HPC when higher contents of BFA particles are incorporated. By analyzing the relationship between the macro properties of concrete strength and the microstructure enhancement mechanism, the importance of BFA in optimizing the HPC performance is explained and the potential of developing the green low carbon concrete with BFA is proposed.