A method of automatically updating a datum is disclosed in the present invention. The method is applied to a signal outputting device and at least one signal receiving device for datum updating transmission. The method includes outputting a detecting signal to detect whether the signal receiving device is located within an effective range of the detecting signal, receiving a reacting signal generated according to the detecting signal, outputting a request of datum transmission when a distance is smaller than a threshold value, and transmitting the datum to the signal receiving device by wireless transmission when the request is allowed, so as to drive the signal receiving device to execute application program according to the updated datum. The detecting signal represents the distance between the signal outputting device and the signal receiving device.