Embodiments of this disclosure provide a power determining method and a terminal device. The method includes: obtaining M target ratios, where each of the target ratios is a ratio of one of M target power values to a maximum target power value, the maximum target power value is a maximum value of the M target power values, the M target power values are receive power values of M logical antenna ports, the M logical antenna ports are logical antenna ports of a target receive antenna of the terminal device, and M is an integer greater than 1; and determining a target receive power value based on the M target ratios, where the target receive power value is a receive power value of the target receive antenna.