The present disclosure relates to the field of communications technologies, and discloses a channel quality information determining method, an apparatus, and a system. The method may include: a terminal device obtains a configured TBS, and/or determines channel quality information of each of K segments of frequency domain resources based on the configured TBS and/or a time domain resource range. A frequency domain resource of the terminal device may be divided into N segments, where both N and K are integers greater than or equal to 1, and K is less than or equal to N. In some embodiments, the configured TBS may be fully considered when the channel quality information is determined, so that a network device properly may determine, based on the channel quality information, a frequency domain resource used to transmit a data packet.