This paper discusses thermal design of a segmented high-temperature superconducting (HTS) magnet depending on geometry of HTS conductors, cooling system (indirect cooling or partial forced flow cooling), cooling techniques and joint resistance. For the purpose, three-dimensional heat conduction analysis was carried out with a finite element code, modeling geometry and operating condition of helical coils in a helical fusion reactor, FFHR as an example. In this analysis, liquid neon was assumed to be used as a coolant at an operating temperature of 25 K. As a heat removal technique for the joint, cooling system of a cryogenic liquid coolant with metal porous media has been proposed and it was also modeled in the heat conduction analysis. The numerical results showed that stainless steel jacket and a low thermal conductivity insulator determine temperature distribution and any cooling techniques cannot contribute to prevent the temperature rise when joint resistance increases in the case of the indirect cooling system. On the other hand, a high performance cooling technique such as metal porous media-inserted channel is effective to reduce temperature rise in the partial force cooling system.