Provided are a method for determining an unavailable resource, a terminal device, and a network device. The method includes: determining, by a terminal device, a first resource corresponding to a first-type Synchronization Signal Block (SSB); and determining, by the terminal device, an unavailable resource for Remaining System Information (RMSI) based on the first resource and a second resource corresponding to the RMSI. The first-type SSB is an SSB transmitted on a non-Synchronization Signal (SS) raster. Based on the first resource corresponding to the first-type SSB and the second resource corresponding to the RMSI, the unavailable resource for the RMSI can be determined. This not only ensures that the RMSI resource mapping can be performed correctly, but also makes system scheduling more flexible. For example, a network device does not have to intentionally avoid collisions between the SSB and a PDCCH and a PDSCH of the RMSI in limited time-frequency resources.