We develop the technique to study the spin wave dynamics of the full-polarized state of the Dzyaloshinskii-Moriya helimagnets by polarized small-angle neutron scattering. We have experimentally proven that the spin waves dispersion in this state has the anisotropic form. We show that the neutron scattering image displays a circle with a certain radius which is centred at the momentum transfer corresponding to the helix wave vector in helimagnetic phase $k_s$, which is oriented along the applied magnetic field ${\bf H}$. The radius of this circle is directly related to the spin-wave stiffness of this system. This scattering depends on the neutron polarization showing the one-handed nature of the spin waves in Dzyaloshinskii-Moriya helimagnets in the full-polarized phase. We show that the spin-wave stiffness $A$ for MnSi helimagnet decreased twice as the temperature increases from zero to the critical temperature $T_c$.