We investigated the influence of back-reflected pump power in a high-power Yb:YAG thin-disk laser. In such lasers, there is usually a fraction of pump power that is not absorbed in the Yb:YAG crystal reflected back via the initial path and incident on the pumping laser diode. In a high-power laser, such a reflection may cause catastrophic optical damage in laser diodes. The unabsorbed pump power was calculated using rate equations. The experimental results are in agreement with those obtained through calculations. Furthermore, two methods for back-reflected pump power suppression were introduced. We have shown that with a thin-film polarizer and quarter wave plate, unabsorbed power incident on the laser diode can be reduced significantly.