The design of the direct-liquid-cooled thin-disk laser is detailedly discussed. It includes the choosing of crystal and coolant in the gain module as well as the design of the channel structure. Moreover, the advantages and disadvantages of two kinds of combination modes are analyzed. In the first mode, the incident angle of the beam and the cutting angle of the crystal should be controlled strictly. The theory calculations and analysis are given. In the second mode, the angles do not need to be specially selected, while the refractive index of the coolant should be close to that of the crystal. In the aspect of pump mode selection, the energy storage and aberration effects both in end-pumped and side-pumped lasers are analyzed. Theoretically, with the use of 10 pieces of Nd∶YLF thin-disks as the gain media and the refractive index matched liquid as the coolant, an average output power larger than 1 kW is achieved at the pump power of 5 kW, corresponding to an optical-optical efficiency larger than 20%. The theoretical analysis is basically consistent with the experimental results.