Abstract: This paper deals with the high power laser slab. The influence of the cooling intensities at its top/bottom sides on its highest temperature and maximum thermal stress has been numerically investigated for the case that its pumped sides are cooled with the conventional technique. The optimization of thermal stress field has been performed. The calculation results show that for any pumped power the highest temperature and maximum thermal stress vary with the heat transfer coefficient of the top/bottom sides and the optimum heat transfer coefficient exists. As compared to the case that the top/bottom surfaces are cooled with the conventional technique,using the optimum heat transfer coefficient maximum thermal stress can decrease 36% and the same effect can be achieved if the top/bottom surface temperature is appropriately controlled.