In the present study, we report novel methods to achieve accurate temperature measurements inside the water droplet at its supercooled state as well as during its freezing process. The temperature measurements were based on the molecular tagging thermometry technique. In order to maintain the nonfreezing state of the phosphorescent tracer complex aqueous solution at a subfreezing temperature, a double-layer temperature control container was designed and fabricated. Then, the calibration between the lifetime and temperature of the phosphorescent tracer complex aqueous solution from 7.5 °C to as low as -6.0 °C was carefully performed. Then, the ice fraction (f) was applied to revise the calibration curve for the ice-liquid mixture. The results indicated that the calibration curve for the pure liquid was suitable for the temperature measurements of the pure phosphorescent triplex solution at the supercooling state, while the revised calibration curve for the ice-liquid mixture was appropriate for the temperature measurements in the water droplet during its freezing stage.