The shunt power capacitor is widely used in power system as the reactive power compensation. To estimate the internal hottest-spot temperature of a running power capacitor, it is put forward the calculate model of the hottest-spot temperature of the capacitor inner, which deals the hottest temperature of capacitor shell as the intermediate variable in the balance of heat change. In service, the dielectric loss causes the capacitor's temperature rise. The internal hottest temperature of a power capacitor is an important factor to damage the dielectric in long running. According to the parameters of one power capacitor, the calculation result of the internal hottest temperature is calculated with different running power capacity under several ambient temperatures. The field temperatures of a capacitor bank are measured using a thermal infrared image instrument. The validity of the calculation model is verified by the measurement data. The ambient temperature proposals are offered to strengthen the running safety of the shunt power capacitor.