Real-time quantitative PCR device was emerging to be an essential platform in precision medicine and other genetic testing applications. The traditional heating method is extremely energy and time-consuming. Novel photothermal conversion based mechanism was reported recently that speed up the system to an unprecedented speed and energy efficiency and sensing of the temperature change became a challenging task. In general, the temperature of the reagent in the tubes was determined according to transform equations from the temperature of the heating block. However, the true temperature in each reagent was difficult to be accurately estimated. In this study, we proposed a new optical method to heat the reagent and integrated the thermocouple, metal tape, and thermal conducting rubber to serve as a composite temperature sensor (CTS) for measuring the radiant temperature from the tube. In addition, the temperature of the reagent was also detected by a thermocouple in the reagent. The relationship between the two temperature measurements was compared. The results showed that the CTS with copper tape was more sensitive and stable than that of other groups. A suitable dimension was determined to be 4 x 3 mm for the copper tape with a 1 mm thickness of the thermal conducting rubber. The mathematical correlation between the two measurements was evaluated and turned out to be 96.2 %. In the future, the CTS outside the tube could be applied to estimate the actual temperature of the reagent in the tube in further PCR applications.