A method and device for measuring body temperature and a smart apparatus are disclosed. The method comprises: when a temperature measuring instruction is received, starting a camera and a first infrared thermometer; photographing a user by using the camera to obtain an image, and identifying the image to determine a part to be measured of the user; acquiring a distance between the first infrared thermometer and the user; when the distance is equal to a preset distance threshold, controlling the first infrared thermometer to measure the temperature of the part to be measured of the user; wherein the preset distance threshold is set according to a focal length of a Fresnel lens of the first infrared thermometer; and according to a preset rule and the measured temperature, determining a value or a value range of the user's body temperature.