The present invention discloses a method for detecting body temperature of a live body. Firstly, an infrared thermometer is used to scan a testee to obtain a plurality of infrared signals. Next, the infrared signals are sequenced to obtain the greatest one thereof. Next, several infrared signals closest to the greatest infrared signal are selected from the sequenced infrared signals. Next, an average infrared signal is worked out from the selected infrared signals. Then, the average infrared signal is converted into a temperature. Thereby, not only the test result is more precise, but also the testee can learn the test result sooner.