The thermal stress of the high-power solid-state laser slab is becoming a more and more serious problem hindering the further increase of the power of the laser, the laser slab can operate safely only if it is efficiently cooled. Based on the non-uniform heat transfer coefficient model, researches are performed on the cooling characteristics of the both side-pumped laser slab with non-uniform heat source by forced convection cooling in a narrow passage. Numerical simulations are conducted to investigate the temperature and thermal stress distributions of the laser slab cooled by forced convection cooling through heat sink. Three kinds of materials are selected to serve as the heat sink, they are the composite slab, sapphire, and diamond, respectively. The calculation results show that neglecting the non-uniformity of the convective heat transfer coefficient causes an underprediction of the temperature and thermal stress of the slab. For the present side-pumped/side-cooled laser slab case, the cooling scheme using the diamond heat sink is the best, that using the sappire heat sink follows, whereas that using the composite slab heat sink does not apply.