A new method of fusion power measurement based on flow neutron-activation analysis has been examined. A single-dimension model has been applied to consider mass transfer and diffusion of radioactive fluid matter being irradiated to a fast neutron flux while traveling through a pipe. This model enables evaluation of a temporal resolution and an achievable sensitivity for the proposed method. The application of light elements B, C, N, O, F, and Ne was analyzed. Numerical simulation has demonstrated that a gas-jet activation system should provide fusion power measurement for ITER-FEAT conditions in the range from 2 to 500 MW with a temporal resolution of 10 ms and 10% accuracy.