In this paper, the tapered multimode plastic optical fiber sensor is used for the experimental study of the early-age hydration process of cement paste. A high reactive power is mixed as a specimen, in which a tapered fiber sensor is embedded to measure the liberated heat (temperature). The sensor characteristics are determined and calibrated by an embedded thermocouple, which have a sensitivity of 0.0293 mV/°C and resolution of ±0.34°C. The experimental studies are carried out for the host specimen with different sizes, various water/cement (w/c) ratios, as well as different ambient temperatures. From the experimental results, the curing temperature and setting time are determined by monitoring the curing temperature curves as the curing progressed. The curing temperature rose with increasing mould sizes, increasing w/c ratio, and increasing ambient temperature. The setting time could be shortened by a smaller size of a specimen, lower w/c ratio, and higher ambient temperature.