A method, a mobile device, and a system for message transmission of a vulnerable road user (VRU) are provided. The method is adapted to a first mobile device of a first VRU and includes the following steps. Whether there exists a target mobile device within a preset range of the first VRU is determined, where the target mobile device is a mobile device that has already broadcast a personal safety message (PSM). When the target mobile device exists within the preset range, whether to broadcast a first PSM is determined according to a relative distance between the target mobile device and an intersection as well as a relative distance between the target mobile device and the first mobile device. When the target mobile device doesn't exist within the preset range, the first PSM is broadcast.