By numerically solving the tow-dimensional time-dependent non-linear Fokker-Planck equations, the distribution functions are calculated and the D-T fusion reactivity are evaluated. For the tritium parallel injection case, the reactivity variation induced by the non-Maxwellian distribution is analyzed. It is pointed out that it is appropriate to define. The calculation resutlts show that η increases from 1 to maximum and then decreases during NBI heating. Either high input power or high beam energy can make η to decrease to a value less than 1. When η decreases to 1, Ｔ∥Ｔ is raised to about 10keV.