This application provides a first device, a second device, a signal transmission method, and a wireless access system. In solutions provided in this application, the first device can process a digital baseband signal into an analog radio frequency signal, and then transmit the analog radio frequency signal to a plurality of second devices through optical carriers. Therefore, no radio frequency circuit needs to be disposed in each second device, to effectively simplify a signal processing procedure of the second device, and reduce power consumption, a size, and costs of the second device. Correspondingly, power consumption and costs of the wireless access system can be significantly reduced; in addition, a quantity of second devices that can be connected to the first device can be increased, to effectively increase a coverage area of the wireless access system. In addition, the first device can directly transmit an analog radio frequency signal through an optoelectronic unit. In comparison with transmission of a digital baseband signal, because no additional control information needs to be transmitted, an effective radio frequency signal transmission bandwidth is improved.