  $\mathsf{Mn_{3}Sn}$ has recently attracted considerable attention as a
magnetic Weyl semimetal exhibiting concomitant transport anomalies at room
temperature. The topology of the electronic bands, their relation to the
magnetic ground state and their nonzero Berry curvature lie at the heart of the
problem. The examination of the full magnetic Hamiltonian reveals otherwise
hidden aspects of these unusual physical properties. Here, we report the full
spin wave spectra of $\mathsf{Mn_{3}Sn}$ measured over a wide momentum - energy
range by the inelastic neutron scattering technique. Using a linear spin wave
theory, we determine a suitable magnetic Hamiltonian which not only explains
the experimental results but also stabilizes the low-temperature helical phase,
consistent with our DFT calculations. The effect of this helical ordering on
topological band structures is further examined using a tight-binding method,
which confirms the elimination of Weyl points in the helical phase. Our work
provides a rare example of the intimate coupling between the electronic and
spin degrees of freedom for a magnetic Weyl semimetal system.
