The invention provides a flashlight for automatically adjusting brightness with the change of an irradiation distance, and discloses a light source for automatically adjusting the brightness with the change of the irradiation distance. The flashlight comprises an ultrasonic distance detection module, a switch, a button, a flashlight body, a snoot, an optical filter, an led lamp, a front fixing ring, a back fixing ring, a circuit board and other parts. The distance detection module can detect the distance between an irradiated object aligned to the light source and the light source in real time and transmit the information to a singlechip through an I/O port, the singlechip receives the signal, outputs PWM waves with different duty ratios, controls a driving module to output different voltages and provides different current for the high-power led lamp through a resistor, in order to adjust the brightness of the light source and automatically adjust the light intensity of the light source with the change of the irradiation distance, so that an illumination effect is guaranteed to be basically unchanged in the case of light source jitter or distance change. The flashlight provided by the invention automatically changes the illumination brightness according to the distance, and meanwhile, the illumination brightness corresponding to the same irradiation distance can also be changed by the button to realize human-friendly adjustment.