A fuel test rig is fabricated by various welding technologies jointing various sensors and the end caps with cladding which inserts the nuclear fuels. To precisely weld the pin-hole which is drilled on the end cap of a nuclear fuel rod, a high-pressure filling system was designed and developed. This welding system consists of mainly a high-pressure chamber, a spot welding jig and a tungsten inert-gas (TIG) welder. The nuclear fuelrod welded with this process should not leak helium gas out of its outside, when it is taken out of the high-pressure chamber. Using this system, the performance tests were performed with TIG spot welding conditions. And the welding characteristics were conformed with welded surfaces and microstructure of end-cap samples. This paper describes not only the composition and function of the high-pressure gas filling system for filling helium gas but also methods and results of the spot welding tests.