In this paper, a comprehensive multiphysics field model is developed to assess the thermal effects of direct-liquid-cooled side-pumped Ti∶Sapphire thin-disk lasers. The thermal analysis of the disks subjected to lateral nonideal pumping revealed that the total reflection transmission of pump light within the disks does not cause considerable temperature fluctuations. In addition, the thermal boundary layer thickness of the cooling fluid is considerably smaller than half of the fluid thickness, ensuring there is no thermal interference between adjacent disks. Furthermore, the thermal safety analysis revealed that, as the thermal power increases, the operational limits of the laser under the prescribed conditions are determined by the solid-liquid interface temperature, rather than the thermal stress on the lamellae. The Ti∶Sapphire lamellae can handle a thermal loading power of up to 4.43 kW, which is 4.92 times greater than that of Nd∶YAG under identical operational parameters. Finally, the analysis of thermal wavefront distortion revealed that, despite the cooling fluid possessing a thermal-optical coefficient tens of times higher than that of the lamellae, the thin thermal boundary layer prevents it from being the dominant factor. The wavefront distortion introduced by the thermal-optical effect of the fluid is balanced by the thermal-optical effect of the lamellae medium and thermal distortion. This suggests that optimizing the design can considerably enhance beam quality.