Hydration reactions of cement-based materials are of great significance to their properties and durability. Various technologies have been investigated to study the hydration processes regarding reaction heat, chemical changes, or microstructures. As a non-destructive chemical analysis technique, Raman spectroscopy provides detailed information about chemical structures, which takes advantage of tracking and monitoring the chemical change during the hydration reaction. In this study, a novel in situ fiber optic Raman probe was utilized to continuously monitor the long-term hydration process of cement clinker stages, from early to late hydration stages and from fresh to hardened state of paste samples. With the remarkable capability of this technique for dry or moist, crystalline or amorphous samples, the hydration process of tricalcium silicate (C3S) pastes with different water-to-solid (w/s) ratios can be monitored from the beginning of the hydration reaction. The main hydration products, especially C-S-H and silicate (CH), have been successfully identified, and there in situ quantitative changes have been continuously monitored. The effect of the w/s ratio on the hydration process of C3S slurries is also discussed. Moreover, the X-Ray Powder Refraction (XRD) results strongly correlate with the Raman spectra of the hydration products, demonstrating the technique's reliability. By comparing with the existing in situ fiber optic Raman spectroscopy technique, the proposed sensor performs a significantly better signal-to-noise ratio (SNR), providing essential aid for future use in the construction field for monitoring and assessing the health and performance of concrete structures.