Cesium atomic fountain clocks are standard frequency signal generators based on quantum transitions in atoms, widely utilized in timekeeping systems and other precision measurement applications. The ultra-low-temperature cesium atomic fountain clock represents an advanced version of this technology, operating with cesium atoms in a liquid nitrogen (80 K) environment. Reducing the surrounding temperature from 300 to 80 K leads to a 187-fold decrease in the blackbody radiation frequency shift and a 79-fold reduction in the associated uncertainty. Additionally, improvements in microwave cavity phase frequency shifts and background collision frequency shifts are observed. The microwave resonant cavity, a critical component of the ultra-low-temperature cesium atomic fountain clock, requires tuning and testing to align its resonant frequency with the cesium atomic transition frequency. Although the cavity is tuned under atmospheric, room-temperature conditions, it functions in a vacuum and ultra-low-temperature environment, where thermal expansion and contraction cause significant parameter variations. To validate the tuning process, it is crucial to replicate the actual working conditions of the cavity experimentally. This paper describes the development of an ultra-low-temperature resonant cavity testing system. Using microwave resonant cavity design theory, the system′s working parameters were calculated, and a finite element model was created to simulate the temperature distribution of the cavity in the ultra-low-temperature environment. The testing system meets all necessary requirements for uniform temperature, vacuum level, and insulation performance. Specifically, it achieves a vacuum level of 10-2 Pa, a temperature tuning range from 78 to 86 K, and a temperature control accuracy of 0.02 K.