Under low temperature environment, the concrete strength develops slowly, which will seriously hinder the construction progress. Traditional early-strength components can not meet the requirements of green and high-performance concrete, combined with the research and application of early-strength accelerators under low temperature is relatively few, and its low-temperature early-strength performance is limited, the early strength development rate of specimens is far lower than that of blank samples at 20 °C, so it is urgent to develop new early-strength component to promote the strength development rate of concrete at low temperature. In this paper, calcium bromide (CaBr2) was used as an early strength component to test the 1-, 3-, 7-, and 28-d compressive strengths of the mortars and cement pastes mixed with 0–1.0% CaBr2 under 5 °C curing. CaBr2 had a significant effect of early strength at low temperature (5 °C), and the strength of the specimens mixed with 1.0% CaBr2 at each age was close to or even exceeded that of the contrast samples cured at 20 °C. The equilibrium method (gravimetric method) was used to study the effect of 1 mol/L of CaBr2 in water on the dissolution of the single ores C3S at different ages. CaBr2 could promote the dissolution of C3S at a low temperature of 5 °C, and the solubility of C3S had an obvious tendency to increase. Further, X-ray diffraction (XRD) analysis, scanning electron microscope (SEM) and energy-dispersive spectrometry (EDS) analysis and mercury infusion porosimetry (MIP) analysis were used to perform microscopic characterization (e.g., product composition, morphology, pore structure, etc.) of the hydration products of the cement single ores or mortars mixed with CaBr2. On this basis, the early-strength mechanism of CaBr2 was discussed. CaBr2 could promote the initial hydration of cement at low temperature (5 °C) and significantly increase the degree and rate of hydration of C3S and C3A single ores, and also generated new type of products, bromine-containing C–S–H gels and bromoaluminate calcium hydrate (Ca4Al2O6Br2·10H2O), and the Ettringite and Ca(OH)2 crystals were mostly formed on the surface of gel products or inside the pores. Large amounts of hydration products were generated, which refined the pore diameter of samples by reducing the most probable pore size and total porosity, thus making the microstructure of samples denser and enhancing the strength of specimens.