Abstract: An automatically inflatable safety device includes an inflatable safety jacket mounted on a user's body and having an inner face formed with an air valve, and an inflation mechanism mounted on the air valve of the safety jacket to supply air into the safety jacket. Thus, the automatically inflatable safety device is available for traffic vehicles applied in the land (such as the motorcycle) and the water (such as the boat or ship), so that the automatically inflatable safety device has an amphibious effect, thereby greatly enhancing the versatility of the automatically inflatable safety device.