This application provides a system information transmission method, and a related device and system. The method includes: receiving, by a network device, a system information request sent by one or more terminals; transmitting, by the network device, system information SI, where SI transmitted for one terminal has a quasi-co-location QCL with a portion of downlink signal blocks, and the portion of downlink signal blocks are a subset of all downlink signal blocks transmitted by the network device; and sending, by the network device, first indication information, where the first indication information indicates a mapping between a downlink signal block and SI, and the downlink signal block has a QCL with the SI corresponding to the downlink signal block. The foregoing solution can reduce system overheads and improve resource utilization.