In this paper, the hydration process of Portland cement at early age was characterised by Fourier transform infrared spectroscopy (FTIR); the results showed that FTIR is a simple and rapid research method in operation, which can reflect the basic variation in the primary cement hydration products. As the hydration reaction proceeded, the peak value corresponding to silicon–oxygen (Si–O) vibrations in the FTIR spectrum shifted from low wavenumbers to high wavenumbers, which not only reflected the polymerisation process of silicon–oxygen tetrahedrons in the calcium–silicate–hydrate gel, but also the hydration reaction speed of cement paste at this stage. This was confirmed by experiment on samples with added superplasticiser. The shifting characteristics of the absorption peaks in the range 800–1025 cm−1 for samples at the ages of 8–12 h were comprehensively investigated in this study. The research results showed that the shifting speed of the characteristic peak for the blank sample was exactly the same as that for the sample reinforced with carbon nanotubes (CNTs). Therefore, the addition of CNT materials into Portland cement did not influence the hydration reaction at the early age. This conclusion was verified by the thermometry test for hydration temperature.