The extended Landau theory about slowing down of electron traversing through matter has been applied to the investigation of slowing down of fast charged particles passing through fusion plasma. It is shown that there are some obvious differences between the solution of Fokker-Planck equation and our results. These differences come from different descriptions for larger energy exchange events in collisions. As a con crete application of the present theory, the energy gain of the fusion reaction produced by mixed deuterium and tritium beam has been calculated and the significance of improving particle confinement time has been shown.