Embodiments of this application disclose a signal transmission method, a related device, and a system. The method may include: receiving first indication information sent by a network device, where the first indication information is used to indicate an uplink resource that is allocated by the network device to a terminal in a first bandwidth; the resource indicated by the first indication information includes an integer quantity of resource blocks evenly distributed in a part or all of the first bandwidth; performing uplink transmission in a detected idle second bandwidth; and sending second indication information to the network device, where the second indication information is used to indicate the second bandwidth. This solution can support flexible bandwidth transmission, and can better adapt to a multi-bandwidth scenario supported by a next-generation new radio technology.