A novel high-power direct-liquid-cooled thin-disk solid-state laser is designed, in which the distributed gain system is composed of tens or hundreds transmission disks by intensive stacking. A special kind of laser cooling liquid flows in the planar micro-channels between gain media, and thus the direct cooling of disks is realized. The thermal stress, the reflective surface deformation and so on caused by the soldering between gain media and the heat-sink in the traditional high power solid-state laser are successfully avoided. In addition, the parameters such as intra-cavity loss and aberration are optimized. The key factors influencing the optical-to-optical conversion efficiency are analyzed, and the methods for controlling laser beam quality are introduced according to the thermal aberration characteristics. A gain module is composed of 20 disks by intensive stacking with a special angle. With these gain modules, a quasi-continuous-wave (QCW) polarized laser with an output power of larger than 9 kW is obtained in both stable and unstable cavities. Moreover, the whole volume of this laser source in laboratory is smaller than 0.4 m 3.